// Copyright Epic Games, Inc. All Rights Reserved.

"use strict";

import { ZenPage } from "./page.js"
import { Fetcher } from "../util/fetcher.js"
import { CbObject } from "../util/compactbinary.js"
import { Table, PropTable } from "../util/widgets.js"
import { make_platform_cell } from "./platform_icons.js"

// Run @p fn and swallow any thrown error, but log it at debug level under
// @p label so the failure isn't completely invisible. Use this in places
// where the failure mode is genuinely "drop the frame and move on" — JSON /
// CB parse failures, optional WebSocket setup, transient send errors. The
// debug-level log keeps normal consoles clean; surface them by enabling
// "Verbose" in DevTools.
function quietly(label, fn)
{
	try { return fn(); }
	catch (e)
	{
		console.debug(`[sessions] ${label}:`, e);
		return undefined;
	}
}

// Dev tools read better with 24h + zero-padded fields; don't defer to the
// browser's default locale which is 12h AM/PM for en-US.
const TIME_OPTS = { hour12: false, hour: "2-digit", minute: "2-digit", second: "2-digit" };
const DATE_OPTS = { year: "numeric", month: "2-digit", day: "2-digit", ...TIME_OPTS };

// UE log levels in ascending order of severity. Each incoming entry's
// level string (lowercased) maps to a numeric rank we can compare against
// the user-selected threshold when filtering. Both the short ("warn") and
// long ("warning") spellings are covered because zencore emits the long
// form but older clients / tests may use the short one.
const LEVEL_RANK = {
	trace:    0,
	debug:    1,
	info:     2,
	warning:  3, warn: 3,
	error:    4, err:  4,
	critical: 5,
};

// Cap on the in-DOM log line count. The server's deque is bounded
// (MaxLogEntries on the C++ side) but the WS push delivers every new
// entry forever during a long Cook, so the browser DOM would grow
// without bound. At 5000 lines the panel still feels live for tail-
// following while keeping the page responsive. Older entries fall off
// the far end on every append.
const MAX_LOG_LINES_IN_DOM = 5000;

// Level-filter dropdown options. `rank` is the minimum rank that survives
// — entries with a lower rank are hidden. -1 disables level filtering.
const LEVEL_FILTER_OPTIONS = [
	{ value: "all",   label: "All levels", rank: -1 },
	{ value: "debug", label: "Debug+",     rank:  1 },
	{ value: "info",  label: "Info+",      rank:  2 },
	{ value: "warn",  label: "Warn+",      rank:  3 },
	{ value: "error", label: "Error+",     rank:  4 },
];

// Double-chevron icons for the expand / collapse panel toggle. Up when
// collapsed (click to grow the log panel upward into the table's space);
// down when expanded (click to shrink back down). currentColor so the
// surrounding button styles control tinting.
const ICON_CHEVRON_UP = '<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><polyline points="6 17 12 11 18 17"/><polyline points="6 11 12 5 18 11"/></svg>';
const ICON_CHEVRON_DOWN = '<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><polyline points="6 7 12 13 18 7"/><polyline points="6 13 12 19 18 13"/></svg>';

function fmt_date(iso)
{
	if (!iso) { return "-"; }
	const d = new Date(iso);
	const now = new Date();
	if (d.getFullYear() === now.getFullYear() && d.getMonth() === now.getMonth() && d.getDate() === now.getDate())
	{
		return d.toLocaleTimeString([], TIME_OPTS);
	}
	return d.toLocaleString([], DATE_OPTS);
}

function fmt_time(iso)
{
	if (!iso) { return ""; }
	return new Date(iso).toLocaleTimeString([], TIME_OPTS);
}

////////////////////////////////////////////////////////////////////////////////
export class Page extends ZenPage
{
	generate_crumbs() {}

	async main()
	{
		this.set_title("sessions");

		this._status = this.get_param("status", "all");

		const section = this.add_section("Sessions");
		const section_dom = section._parent.inner();
		section_dom.classList.add("sessions-section");
		// The "Sessions" nav item in the banner already identifies the page;
		// drop the auto-generated section heading so it doesn't duplicate.
		const heading = section_dom.querySelector(":scope > h1, :scope > h2");
		if (heading) { heading.remove(); }

		const query = (this._status === "ended" || this._status === "all") ? "?status=" + this._status : "";
		const data = await new Fetcher().resource("/sessions/" + query).json();
		const sessions = data.sessions || [];
		this._self_id = data.self_id || null;

		// Flat vertical layout: header row, then table, then the bottom panel
		// (tabs for log and metadata). All session-level info that used to
		// live in a side panel is now shown inline in the table columns.
		this._init_status_tabs(section, this._status);
		this._table_host = section.tag().classify("sessions-table");
		this._selected_id = this._self_id;
		this._selected_row = null;
		this._page_size = 10;
		this._page = 0;
		this._text_filter = "";
		this._sort_key = "created_at";
		this._sort_asc = false;

		this._panel = section.tag().classify("sessions-log-panel");
		this._panel.inner().style.display = "none";
		this._active_tab = "log";
		this._log_expanded = false;
		// Persist the level threshold across session selections so users
		// don't have to re-pick after clicking a different session.
		this._log_min_level = -1;
		this._log_min_level_name = "all";
		this._collapsed_session_groups = new Set();

		this._render_sessions(sessions);
		this._connect_ws();
	}

	_render_sessions(sessions)
	{
		const status = this._status;

		// Clear existing table content (and any prior pager contents; the
		// pager lives in the header row but its state depends on what we're
		// about to render).
		this._table_host.inner().replaceChildren();
		if (this._pager_host)
		{
			this._pager_host.replaceChildren();
		}

		this._last_sessions = sessions;

		if (sessions.length === 0)
		{
			const labels = { active: "No active sessions.", ended: "No ended sessions.", all: "No sessions." };
			this._table_host.tag().classify("empty-state").text(labels[status] || labels.all);
			this._selected_row = null;
			return;
		}

		// Apply the text filter (case-insensitive substring match across the
		// session fields a user is likely to scan for: id, appname, mode).
		const filter = this._text_filter;
		let filtered = filter
			? sessions.filter(s => {
				const haystack = [s.id, s.appname, s.mode].filter(v => v).join(" ").toLowerCase();
				return haystack.includes(filter);
			})
			: sessions;

		if (filtered.length === 0)
		{
			this._table_host.tag().classify("empty-state").text("No sessions match the filter.");
			this._selected_row = null;
			return;
		}

		// When the log panel is expanded, collapse the row set to just the
		// selected session so the log gets the maximum vertical real estate.
		// The expand toggle lives in the panel header — see _show_session_panel.
		if (this._log_expanded && this._selected_id)
		{
			const selected = sessions.find(s => s.id === this._selected_id);
			if (selected) { filtered = [selected]; }
		}

		// Column specs carry both the header label and how to extract the
		// real sort value so date columns compare chronologically rather
		// than by locale-formatted text.
		const str_val  = (field) => (s) => (s[field] || "").toLowerCase();
		const date_val = (field) => (s) => s[field] ? new Date(s[field]).getTime() : 0;

		const common = [
			{ name: "appname",  key: "appname",    kind: "str",  get: str_val("appname") },
			{ name: "mode",     key: "mode",       kind: "str",  get: str_val("mode") },
			{ name: "platform", key: "platform",   kind: "str",  get: str_val("platform") },
			{ name: "id",       key: "id",         kind: "str",  get: str_val("id") },
			{ name: "created",  key: "created_at", kind: "date", get: date_val("created_at") },
		];
		let last_col;
		if (status === "all")
		{
			last_col = { name: "last activity", key: "last_activity", kind: "date",
				get: s => new Date(s.ended_at || s.updated_at || 0).getTime() };
		}
		else if (status === "ended")
		{
			last_col = { name: "ended", key: "ended_at", kind: "date", get: date_val("ended_at") };
		}
		else
		{
			last_col = { name: "updated", key: "updated_at", kind: "date", get: date_val("updated_at") };
		}
		const col_specs = [...common, last_col];

		// Pick the active sort column (fall back to created_at if the current
		// sort key isn't in this tab's column set — e.g. switching from "all"
		// back to "ended" after sorting by last_activity).
		const sort_col = col_specs.find(c => c.key === this._sort_key) || col_specs.find(c => c.key === "created_at");
		const dir = this._sort_asc ? 1 : -1;
		const compare_sessions = (a, b) => {
			const av = sort_col.get(a), bv = sort_col.get(b);
			if (av < bv) return -1 * dir;
			if (av > bv) return  1 * dir;
			return 0;
		};
		const grouped = this._build_session_groups(filtered);
		grouped.parents.sort(compare_sessions);
		for (const parent of grouped.parents)
		{
			grouped.children.get(parent.id)?.sort(compare_sessions);
		}

		const total = grouped.parents.length;
		const page_count = this._page_size > 0 ? Math.ceil(total / this._page_size) : 1;
		if (this._page >= page_count) { this._page = Math.max(0, page_count - 1); }
		const start = this._page_size > 0 ? this._page * this._page_size : 0;
		const visible = this._page_size > 0 ? grouped.parents.slice(start, start + this._page_size) : grouped.parents;

		const column_names = col_specs.map(c => c.name);
		const table = new Table(this._table_host, column_names, Table.Flag_FitLeft, -1);

		// Attach header click handlers + active-column indicator.
		const zen_table = this._table_host.inner().querySelector(".zen_table");
		const header_elem = zen_table ? zen_table.firstElementChild : null;
		if (header_elem)
		{
			const header_cells = header_elem.children;
			for (let i = 0; i < col_specs.length; i++)
			{
				const col = col_specs[i];
				const cell = header_cells[i];
				if (!cell) { continue; }
				cell.style.cursor = "pointer";
				cell.style.userSelect = "none";
				if (col.key === sort_col.key)
				{
					cell.textContent = col.name + (this._sort_asc ? " \u25B2" : " \u25BC");
					cell.classList.add("sessions-sort-active");
				}
				cell.addEventListener("click", () => {
					if (this._sort_key === col.key)
					{
						this._sort_asc = !this._sort_asc;
					}
					else
					{
						this._sort_key = col.key;
						// New column defaults: dates start descending (newest
						// first — the natural reading for timestamps); string
						// columns start ascending (A→Z).
						this._sort_asc = col.kind !== "date";
					}
					this._page = 0;
					this._render_sessions(this._last_sessions);
				});
			}
		}

		let new_selected_row = null;
		let new_selected_session = null;

		const render_session_row = (session, is_child = false, child_count = 0) => {
			const created = fmt_date(session.created_at);
			const updated = fmt_date(session.updated_at);
			const ended = fmt_date(session.ended_at);

			const mode = session.mode || "-";

			const appname  = session.appname || "-";
			const platform = session.platform || "";
			const full_id  = session.id || "";
			// Elide the middle of the 24-char OID so the column stays narrow;
			// the full id is still available as a tooltip on the cell below.
			const id_display = full_id.length > 12
				? full_id.slice(0, 8) + "\u2026" + full_id.slice(-4)
				: (full_id || "-");

			let row_values;
			if (status === "all")
			{
				const last_activity = session.ended_at ? ended : updated;
				row_values = [appname, mode, platform, id_display, created, last_activity];
			}
			else if (status === "ended")
			{
				row_values = [appname, mode, platform, id_display, created, ended];
			}
			else
			{
				row_values = [appname, mode, platform, id_display, created, updated];
			}

			const row = table.add_row(...row_values);

			// Swap the platform cell's text for a recognizable icon. Sort
			// already runs on session.platform so the cell content doesn't
			// affect ordering.
			const platform_cell = row.get_cell(2).inner();
			platform_cell.replaceChildren(make_platform_cell(platform));

			if (full_id)
			{
				row.get_cell(3).inner().title = full_id;
			}

			// Indicator layout in the appname cell: [group toggle] [child elbow]
			// [dot] appname [this] [log]. The pills sit after the name so their
			// widths don't push names around and misalign the column across rows.
			const appname_cell = row.get_cell(0);
			if (child_count > 0)
			{
				const collapsed = this._collapsed_session_groups.has(session.id);
				const toggle = document.createElement("button");
				toggle.type = "button";
				toggle.className = "sessions-group-toggle";
				toggle.textContent = collapsed ? "\u25B8" : "\u25BE";
				toggle.title = collapsed ? "Expand child sessions" : "Collapse child sessions";
				toggle.addEventListener("click", (ev) => {
					ev.stopPropagation();
					if (collapsed)
					{
						this._collapsed_session_groups.delete(session.id);
					}
					else
					{
						this._collapsed_session_groups.add(session.id);
					}
					this._render_sessions(this._last_sessions);
				});
				appname_cell.inner().prepend(toggle);
			}
			else if (is_child)
			{
				const spacer = document.createElement("span");
				spacer.className = "sessions-group-child-spacer";
				spacer.textContent = "\u2514";
				appname_cell.inner().prepend(spacer);
			}
			else
			{
				const spacer = document.createElement("span");
				spacer.className = "sessions-group-toggle-spacer";
				appname_cell.inner().prepend(spacer);
			}

			if (status === "all" && !session.ended_at)
			{
				const dot = document.createElement("span");
				dot.className = "health-dot health-green";
				dot.style.marginRight = "6px";
				dot.style.width = "8px";
				dot.style.height = "8px";
				dot.title = "active";
				appname_cell.inner().insertBefore(dot, appname_cell.inner().firstChild.nextSibling);
			}

			if (this._self_id && session.id === this._self_id)
			{
				const pill = document.createElement("span");
				pill.className = "sessions-pill sessions-self-pill";
				pill.textContent = "this";
				appname_cell.inner().appendChild(pill);
			}

			if (session.log_count)
			{
				const log_pill = document.createElement("span");
				log_pill.className = "sessions-pill sessions-log-indicator-pill";
				log_pill.textContent = "log";
				log_pill.title = session.log_count + " log entr" + (session.log_count === 1 ? "y" : "ies");
				appname_cell.inner().appendChild(log_pill);
			}

			const row_elem = row.inner();
			if (is_child)
			{
				for (const cell of row_elem.children)
				{
					cell.classList.add("sessions-child-row");
				}
			}

			// Restore selection
			if (this._selected_id && session.id === this._selected_id)
			{
				new_selected_row = row;
				new_selected_session = session;
			}

			// Table rows use display:contents so we attach click to each cell
			for (const cell of row_elem.children)
			{
				cell.style.cursor = "pointer";
				cell.addEventListener("click", () => this._select_session(row, session));
			}
		};

		const render_session_tree = (session, is_child = false) => {
			const children = grouped.children.get(session.id) || [];
			render_session_row(session, is_child, children.length);
			if (!this._collapsed_session_groups.has(session.id))
			{
				for (const child of children)
				{
					render_session_tree(child, true);
				}
			}
		};

		for (const session of visible)
		{
			render_session_tree(session);
		}

		this._selected_row = null;
		if (new_selected_row)
		{
			this._select_session(new_selected_row, new_selected_session);
		}

		this._render_pager(total, page_count);
	}

	_build_session_groups(sessions)
	{
		const by_id = new Map();
		for (const session of sessions)
		{
			if (session.id)
			{
				by_id.set(session.id, session);
			}
		}

		const parents = [];
		const children = new Map();
		for (const session of sessions)
		{
			const parent_id = session.parent_session_id;
			if (parent_id && by_id.has(parent_id) && parent_id !== session.id)
			{
				let group = children.get(parent_id);
				if (!group)
				{
					group = [];
					children.set(parent_id, group);
				}
				group.push(session);
			}
			else
			{
				parents.push(session);
			}
		}

		// Keep the currently selected child visible when live updates rebuild the
		// table by automatically expanding the group that contains it.
		if (this._selected_id)
		{
			for (const [parent_id, group] of children)
			{
				if (group.some(s => s.id === this._selected_id))
				{
					this._collapsed_session_groups.delete(parent_id);
					break;
				}
			}
		}

		return { parents, children };
	}

	// Shared button.history-tab builder used for both the pager arrows and
	// the status-mode tab strip. opts: { active?, disabled?, on_click? }.
	_make_history_tab(label, opts)
	{
		const btn = document.createElement("button");
		btn.className = "history-tab";
		btn.textContent = label;
		if (opts.active) { btn.classList.add("active"); }
		if (opts.disabled) { btn.disabled = true; }
		if (opts.on_click && !opts.disabled)
		{
			btn.addEventListener("click", opts.on_click);
		}
		return btn;
	}

	_render_pager(total, page_count)
	{
		if (!this._pager_host)
		{
			return;
		}
		this._pager_host.replaceChildren();
		if (!(this._page_size > 0 && total > this._page_size))
		{
			return;
		}

		this._pager_host.appendChild(this._make_history_tab("\u25C0", {
			disabled: this._page === 0,
			on_click: () => {
				this._page--;
				this._render_sessions(this._last_sessions);
			},
		}));

		const label = document.createElement("span");
		label.className = "sessions-pager-label";
		label.textContent = `${this._page + 1} / ${page_count}`;
		this._pager_host.appendChild(label);

		this._pager_host.appendChild(this._make_history_tab("\u25B6", {
			disabled: this._page >= page_count - 1,
			on_click: () => {
				this._page++;
				this._render_sessions(this._last_sessions);
			},
		}));
	}

	_filter_sessions(all_sessions)
	{
		if (this._status === "active")
		{
			return all_sessions.filter(s => !s.ended_at);
		}
		if (this._status === "ended")
		{
			return all_sessions.filter(s => s.ended_at);
		}
		return all_sessions;
	}

	_connect_ws()
	{
		quietly("ws connect", () => {
			const proto = location.protocol === "https:" ? "wss:" : "ws:";
			const ws = new WebSocket(`${proto}//${location.host}/sessions/ws`);
			// Log-push frames arrive as compact-binary over binary frames;
			// asking for ArrayBuffer (default in modern browsers, explicit
			// here) lets us feed the bytes straight into our CB parser.
			ws.binaryType = "arraybuffer";
			this._ws_paused = quietly("ws-paused storage read", () =>
				localStorage.getItem("zen-ws-paused") === "true") === true;
			document.addEventListener("zen-ws-toggle", (e) => {
				this._ws_paused = e.detail.paused;
			});

			ws.onmessage = (ev) => {
				if (this._ws_paused)
				{
					return;
				}
				// Two transports share this socket:
				//   - text/JSON: the session-list snapshots broadcast on a
				//     timer (untyped for backward compat).
				//   - binary/CB: event-driven log deltas, stamped with
				//     type="log" so future frame types can be added.
				if (typeof ev.data === "string")
				{
					quietly("ws json frame", () => {
						const data = JSON.parse(ev.data);
						if (data.self_id) { this._self_id = data.self_id; }
						const all_sessions = data.sessions || [];
						const filtered = this._filter_sessions(all_sessions);
						this._render_sessions(filtered);
					});
				}
				else if (ev.data instanceof ArrayBuffer)
				{
					quietly("ws cb frame", () => {
						const bytes = new Uint8Array(ev.data);
						// CbObject extends CbFieldView, not CbObjectView —
						// to_js_object() lives on CbObjectView.prototype.
						// Bridge via as_object() which wraps the field as
						// a view of the same underlying bytes.
						const frame = new CbObject(bytes).as_object().to_js_object();
						this._handle_ws_frame(frame);
					});
				}
			};

			ws.onopen = () => {
				// Resubscribe after a (re)connect if a session is already
				// selected, so live tailing resumes without a reselect.
				this._resubscribe_log();
			};
			ws.onclose = () => { this._ws = null; };
			ws.onerror = () => { ws.close(); };

			this._ws = ws;
		});
	}

	_handle_ws_frame(frame)
	{
		if (frame && frame.type === "log")
		{
			// Guard against stale deltas that arrive after the user has
			// switched sessions — the server does its best but there's
			// always a window.
			if (frame.session !== this._log_session_id)
			{
				return;
			}
			if (typeof frame.cursor === "number" && frame.cursor < this._log_cursor)
			{
				// Cursor regressed (session reset while we were subscribed).
				this._resync_log_from_zero();
				return;
			}
			this._log_cursor = frame.cursor || this._log_cursor;
			if (Array.isArray(frame.entries) && frame.entries.length > 0)
			{
				this._append_log_entries(frame.entries);
			}
		}
	}

	// Wipe the panel and re-replay the log from cursor 0, then re-
	// subscribe so the WS feeds deltas from the fresh tail. Used both
	// when the WS frame reports a cursor regression (session reset
	// while we were subscribed) and when an HTTP refetch sees the same.
	// Returns the underlying fetch promise so callers can await if they
	// need ordering guarantees.
	_resync_log_from_zero()
	{
		this._log_cursor = 0;
		if (this._log_body) { this._log_body.replaceChildren(); }
		return this._fetch_log().then(() => this._subscribe_log());
	}

	_ws_send(obj)
	{
		const ws = this._ws;
		if (!ws || ws.readyState !== WebSocket.OPEN) { return false; }
		return quietly("ws send", () => { ws.send(JSON.stringify(obj)); return true; }) === true;
	}

	_subscribe_log()
	{
		if (!this._log_session_id) { return; }
		// Don't subscribe until the initial replay has resolved — the
		// cursor is stale 0 until then and the server would flush the
		// entire history we're about to fetch via HTTP, duplicating
		// every line in the DOM.
		if (!this._log_fetch_done) { return; }
		this._ws_send({ type: "sub_log", session: this._log_session_id, cursor: this._log_cursor | 0 });
	}

	_unsubscribe_log()
	{
		this._ws_send({ type: "unsub_log" });
	}

	// Called on ws.onopen to restore the subscription after a reconnect.
	_resubscribe_log()
	{
		this._subscribe_log();
	}

	_init_status_tabs(host, active_status)
	{
		const row = document.createElement("div");
		row.className = "sessions-header-row";
		host.tag().inner().appendChild(row);

		const tabs_el = document.createElement("div");
		tabs_el.className = "history-tabs";
		row.appendChild(tabs_el);

		const make_tab = (label, mode) => {
			tabs_el.appendChild(this._make_history_tab(label, {
				active: mode === active_status,
				on_click: mode === active_status ? null : () => {
					this.set_param("status", mode);
					this.reload();
				},
			}));
		};

		make_tab("Active", "active");
		make_tab("Ended", "ended");
		make_tab("All", "all");

		const filter_input = document.createElement("input");
		filter_input.type = "search";
		filter_input.className = "sessions-list-filter";
		filter_input.placeholder = "Filter\u2026";
		filter_input.autocomplete = "off";
		filter_input.spellcheck = false;
		filter_input.addEventListener("input", () => {
			this._text_filter = filter_input.value.toLowerCase().trim();
			this._page = 0;
			if (this._last_sessions)
			{
				this._render_sessions(this._last_sessions);
			}
		});
		row.appendChild(filter_input);

		// Right-aligned pager host; populated per-render in _render_sessions
		// so the arrows don't live inside the table (where they'd shift
		// vertically as the table grows/shrinks between pages).
		const spacer = document.createElement("span");
		spacer.style.flex = "1";
		row.appendChild(spacer);

		this._pager_host = document.createElement("div");
		this._pager_host.className = "sessions-header-pager";
		row.appendChild(this._pager_host);
	}

	_session_detail_metadata(session)
	{
		const details = {
			status: session.ended_at ? "ended" : "active",
			session_id: session.id || "-",
			parent_session_id: session.parent_session_id || "-",
			appname: session.appname || "-",
			mode: session.mode || "-",
			platform: session.platform || "-",
			pid: session.pid || "-",
			jobid: session.jobid || "-",
			created_at: session.created_at ? fmt_date(session.created_at) : "-",
			updated_at: session.updated_at ? fmt_date(session.updated_at) : "-",
		};
		if (session.ended_at)
		{
			details.ended_at = fmt_date(session.ended_at);
		}
		if (session.log_count)
		{
			details.log_count = session.log_count;
		}
		return details;
	}

	_select_session(row, session)
	{
		const changed = (this._selected_id !== session.id) || !this._log_session_id;

		// Deselect previous
		if (this._selected_row)
		{
			for (const cell of this._selected_row.inner().children)
			{
				cell.classList.remove("sessions-selected");
			}
		}

		this._selected_row = row;
		this._selected_id = session.id;
		for (const cell of row.inner().children)
		{
			cell.classList.add("sessions-selected");
		}

		// Rebuild the bottom panel only when the selection actually changes.
		if (changed)
		{
			this._show_session_panel(session);
		}
	}

	_show_session_panel(session)
	{
		// Unsubscribe from the previous session's log stream (if any)
		// before we switch. The server treats a subsequent sub_log as a
		// replacement, but an explicit unsub makes the intent clear and
		// stops any in-flight pushes that could arrive as we're wiping
		// the panel.
		if (this._log_session_id && this._log_session_id !== session.id)
		{
			this._unsubscribe_log();
		}

		this._log_session_id = session.id;
		this._log_cursor = 0;	// monotonic cursor for incremental fetching
		this._log_fetch_done = false;	// gates _subscribe_log until replay resolves
		this._log_follow = true;
		this._log_newest_first = true;
		this._log_filter = "";

		this._panel.inner().style.display = "";
		this._panel.inner().replaceChildren();

		// Header with tab strip, filter, and log-view controls
		const header = document.createElement("div");
		header.className = "sessions-log-header";

		const tabs = document.createElement("div");
		tabs.className = "sessions-panel-tabs";
		header.appendChild(tabs);

		const log_tab = document.createElement("button");
		log_tab.type = "button";
		log_tab.className = "sessions-panel-tab";
		log_tab.textContent = "Log";
		tabs.appendChild(log_tab);

		const meta_tab = document.createElement("button");
		meta_tab.type = "button";
		meta_tab.className = "sessions-panel-tab";
		meta_tab.textContent = "Metadata";
		tabs.appendChild(meta_tab);

		// Spacer sits between the tab strip and the right-hand controls so
		// the Expand button stays flush right on both tabs (log_controls is
		// hidden on Metadata — if the spacer lived there too, the button
		// would jump left).
		const spacer = document.createElement("span");
		spacer.className = "sessions-log-spacer";
		header.appendChild(spacer);

		// Log-only controls: filter, newest-first, follow. Hidden when the
		// Metadata tab is active since they don't apply there.
		const log_controls = document.createElement("span");
		log_controls.className = "sessions-log-controls";
		header.appendChild(log_controls);

		// Level filter: hides entries below the selected severity. Sits
		// before the text filter since level is a coarser cut than text.
		const level_select = document.createElement("select");
		level_select.className = "sessions-log-level-filter";
		level_select.title = "Hide log entries below this severity level";
		for (const opt of LEVEL_FILTER_OPTIONS)
		{
			const o = document.createElement("option");
			o.value = opt.value;
			o.textContent = opt.label;
			level_select.appendChild(o);
		}
		level_select.value = this._log_min_level_name;
		level_select.addEventListener("change", () => {
			this._log_min_level_name = level_select.value;
			const opt = LEVEL_FILTER_OPTIONS.find(o => o.value === level_select.value);
			this._log_min_level = opt ? opt.rank : -1;
			this._apply_log_filter();
		});
		log_controls.appendChild(level_select);

		const filter_input = document.createElement("input");
		filter_input.type = "text";
		filter_input.className = "sessions-log-filter";
		filter_input.placeholder = "Filter\u2026";
		filter_input.addEventListener("input", () => {
			this._log_filter = filter_input.value.toLowerCase();
			this._apply_log_filter();
		});
		log_controls.appendChild(filter_input);

		const order_btn = document.createElement("button");
		order_btn.className = "history-tab active";
		order_btn.textContent = "Newest first";
		order_btn.addEventListener("click", () => {
			this._log_newest_first = !this._log_newest_first;
			order_btn.classList.toggle("active", this._log_newest_first);
			this._reorder_log();
		});
		log_controls.appendChild(order_btn);

		const follow_btn = document.createElement("button");
		follow_btn.className = "history-tab active";
		follow_btn.textContent = "Follow";
		follow_btn.addEventListener("click", () => {
			this._log_follow = !this._log_follow;
			follow_btn.classList.toggle("active", this._log_follow);
			if (this._log_follow && this._log_body)
			{
				this._scroll_to_follow();
			}
		});
		log_controls.appendChild(follow_btn);
		this._log_follow_btn = follow_btn;

		// Expand / collapse toggle: applies to the whole page layout (table
		// vs log panel balance) so it lives outside log_controls and stays
		// visible on both tabs. Double-chevron direction mirrors the way
		// the panel grows — up when there's room to expand, down when
		// expanded and ready to collapse back.
		const expand_btn = document.createElement("button");
		expand_btn.type = "button";
		expand_btn.className = "history-tab sessions-panel-toggle";
		const refresh_toggle = () => {
			expand_btn.innerHTML = this._log_expanded ? ICON_CHEVRON_DOWN : ICON_CHEVRON_UP;
			expand_btn.title = this._log_expanded
				? "Restore the sessions table"
				: "Collapse the sessions table to focus on this session's log";
			expand_btn.setAttribute("aria-label", this._log_expanded ? "Collapse log panel" : "Expand log panel");
			expand_btn.classList.toggle("active", this._log_expanded);
		};
		refresh_toggle();
		expand_btn.addEventListener("click", () => {
			this._log_expanded = !this._log_expanded;
			refresh_toggle();
			if (this._last_sessions) { this._render_sessions(this._last_sessions); }
		});
		header.appendChild(expand_btn);

		this._panel.inner().appendChild(header);

		// Log body
		const log_body = document.createElement("div");
		log_body.className = "sessions-log-body";
		log_body.addEventListener("scroll", () => {
			const at_follow_edge = this._log_newest_first
				? (log_body.scrollTop <= 4)
				: (log_body.scrollTop + log_body.clientHeight >= log_body.scrollHeight - 4);
			if (this._log_follow !== at_follow_edge)
			{
				this._log_follow = at_follow_edge;
				this._log_follow_btn.classList.toggle("active", this._log_follow);
			}
		});
		this._panel.inner().appendChild(log_body);
		this._log_body = log_body;

		// Metadata/details body. Keep polling running regardless of which tab is
		// visible so cursors stay fresh. Free-form metadata gets the primary
		// left-hand panel; core session fields sit beside it on the right.
		// Use .tag() so child panels are real Components — PropTable reaches into
		// its parent's DOM element through the Component API.
		const meta_body_widget = this._panel.tag().classify("sessions-metadata-body");
		const meta_body		   = meta_body_widget.inner();
		const meta_layout	  = meta_body_widget.tag().classify("sessions-metadata-layout");
		const metadata_panel  = meta_layout.tag().classify("sessions-metadata-panel");
		const details_panel   = meta_layout.tag().classify("sessions-metadata-panel").classify("sessions-metadata-core-panel");

		const metadata_heading = document.createElement("div");
		metadata_heading.className = "sessions-metadata-heading";
		metadata_heading.textContent = "Metadata";
		metadata_panel.inner().appendChild(metadata_heading);

		const has_metadata = session.metadata && Object.keys(session.metadata).length > 0;
		if (has_metadata)
		{
			const meta_props = new PropTable(metadata_panel);
			meta_props.add_object(session.metadata);
		}
		else
		{
			const empty = document.createElement("div");
			empty.className = "sessions-log-empty";
			empty.textContent = "No metadata.";
			metadata_panel.inner().appendChild(empty);
		}

		const details_heading = document.createElement("div");
		details_heading.className = "sessions-metadata-heading";
		details_heading.textContent = "Session Information";
		details_panel.inner().appendChild(details_heading);

		const detail_props = new PropTable(details_panel);
		detail_props.add_object(this._session_detail_metadata(session));

		const set_active_tab = (tab) => {
			this._active_tab = tab;
			const is_log = tab === "log";
			log_tab.classList.toggle("active", is_log);
			meta_tab.classList.toggle("active", !is_log);
			log_body.style.display = is_log ? "" : "none";
			meta_body.style.display = is_log ? "none" : "";
			log_controls.style.display = is_log ? "" : "none";
		};
		log_tab.addEventListener("click", () => set_active_tab("log"));
		meta_tab.addEventListener("click", () => set_active_tab("meta"));
		set_active_tab(this._active_tab || "log");

		// Initial HTTP fetch gives us the full history in one shot; after
		// it returns we hand off to the WebSocket for live deltas. Mark
		// the panel as "fetch done" so _resubscribe_log (fired from
		// ws.onopen) can avoid racing a too-early subscribe with
		// cursor=0 that'd cause a duplicate flush. No more setInterval
		// — pushes arrive the moment an entry is appended. See
		// _handle_ws_frame.
		this._fetch_log().then(() => {
			this._log_fetch_done = true;
			this._subscribe_log();
		});
	}

	async _fetch_log()
	{
		if (!this._log_session_id)
		{
			return;
		}

		try
		{
			const data = await new Fetcher()
				.resource("/sessions/" + this._log_session_id + "/log")
				.param("cursor", String(this._log_cursor))
				.json();

			const entries = data.entries || [];
			const cursor = data.cursor || 0;
			const count = data.count || 0;

			if (cursor < this._log_cursor)
			{
				// Cursor went backwards — session was reset. Resync via
				// the shared helper so the WS-frame and HTTP-fetch paths
				// stay in lockstep.
				await this._resync_log_from_zero();
				return;
			}

			this._log_cursor = cursor;

			if (entries.length > 0)
			{
				this._append_log_entries(entries);
			}
			else if (count === 0 && !this._log_body.hasChildNodes())
			{
				this._show_log_empty();
			}
		}
		catch (e)
		{
			// quietly() can't wrap an awaited body, so the catch is open-coded
			// here — same debug-log policy as the sync paths above.
			console.debug("[sessions] fetch log:", e);
		}
	}

	_show_log_empty()
	{
		const body = this._log_body;
		if (!body)
		{
			return;
		}
		body.replaceChildren();
		const empty = document.createElement("div");
		empty.className = "sessions-log-empty";
		empty.textContent = "No log entries.";
		body.appendChild(empty);
	}

	_append_log_entries(entries)
	{
		const body = this._log_body;
		if (!body)
		{
			return;
		}

		// Remove the "No log entries." placeholder if present
		const empty_el = body.querySelector(".sessions-log-empty");
		if (empty_el)
		{
			empty_el.remove();
		}

		if (this._log_newest_first)
		{
			// Prepend in reverse so newest ends up at the top
			const first_child = body.firstChild;
			for (let i = entries.length - 1; i >= 0; i--)
			{
				body.insertBefore(this._create_log_line(entries[i]), first_child);
			}
		}
		else
		{
			for (const entry of entries)
			{
				body.appendChild(this._create_log_line(entry));
			}
		}

		// Cap DOM size. Drop the oldest lines from whichever end of the
		// container holds them — that's the bottom in newest-first mode,
		// the top in oldest-first mode. The user can no longer scroll
		// further back than MAX_LOG_LINES_IN_DOM until they switch
		// sessions and replay from cursor 0.
		const overflow = body.children.length - MAX_LOG_LINES_IN_DOM;
		if (overflow > 0)
		{
			if (this._log_newest_first)
			{
				for (let i = 0; i < overflow; i++)
				{
					body.removeChild(body.lastElementChild);
				}
			}
			else
			{
				for (let i = 0; i < overflow; i++)
				{
					body.removeChild(body.firstElementChild);
				}
			}
		}

		if (this._log_follow)
		{
			this._scroll_to_follow();
		}
	}

	_scroll_to_follow()
	{
		if (!this._log_body)
		{
			return;
		}
		if (this._log_newest_first)
		{
			this._log_body.scrollTop = 0;
		}
		else
		{
			this._log_body.scrollTop = this._log_body.scrollHeight;
		}
	}

	_reorder_log()
	{
		const body = this._log_body;
		if (!body)
		{
			return;
		}

		// Reverse all log line elements in place
		const lines = Array.from(body.querySelectorAll(".sessions-log-line"));
		for (const line of lines)
		{
			body.prepend(line);
		}

		this._scroll_to_follow();
	}

	_create_log_line(entry)
	{
		const line = document.createElement("div");
		line.className = "sessions-log-line";

		const ts = document.createElement("span");
		ts.className = "sessions-log-ts";
		ts.textContent = fmt_time(entry.timestamp);
		line.appendChild(ts);

		if (entry.level)
		{
			const key = entry.level.toLowerCase();
			const rank = LEVEL_RANK[key];
			// Stamp the rank so _line_passes_filters can check it later
			// without re-parsing the text. Unknown levels leave it unset.
			if (rank !== undefined) { line.dataset.levelRank = String(rank); }
			const lvl = document.createElement("span");
			lvl.className = "sessions-log-level sessions-log-level-" + key;
			lvl.textContent = entry.level;
			line.appendChild(lvl);
		}

		// Always render the logger column (even if empty) so the message
		// column stays aligned across rows whether or not a category is set.
		const cat = document.createElement("span");
		cat.className = "sessions-log-logger";
		if (entry.logger)
		{
			cat.textContent = entry.logger;
			cat.title = entry.logger;
		}
		line.appendChild(cat);

		// Marker for UE_LOGFMT structured entries. The server pre-renders
		// `format` against `fields` into `message`, but both raw pieces ride
		// along in the JSON so we can flag them visually and let a future UI
		// hook in for field-level drill-down. Tooltip shows the raw template
		// plus the arguments bag so you can see exactly what UE sent.
		if (entry.format)
		{
			const fmt_marker = document.createElement("span");
			fmt_marker.className = "sessions-log-fmt-marker";
			fmt_marker.textContent = "{\u2026}";
			let tooltip = "format: " + entry.format;
			if (entry.fields && Object.keys(entry.fields).length > 0)
			{
				try
				{
					tooltip += "\nfields: " + JSON.stringify(entry.fields, null, 2);
				}
				catch (_e)
				{
					// Shouldn't happen for server-produced JSON, but guard
					// against self-referential structures just in case.
					tooltip += "\nfields: <unserializable>";
				}
			}
			fmt_marker.title = tooltip;
			line.appendChild(fmt_marker);
		}

		if (entry.message)
		{
			const msg = document.createElement("span");
			msg.className = "sessions-log-msg";
			msg.textContent = entry.message;
			line.appendChild(msg);
		}

		if (!this._line_passes_filters(line))
		{
			line.style.display = "none";
		}

		return line;
	}

	// Shared predicate for both the initial render (_create_log_line) and
	// full sweeps (_apply_log_filter). Keeps the two paths in sync.
	_line_passes_filters(line)
	{
		if (this._log_min_level >= 0)
		{
			const rank_str = line.dataset.levelRank;
			// Entries without a known level rank pass through — they may
			// carry info that shouldn't be silently dropped (e.g. the
			// synthetic "session ended" line or legacy entries without a
			// level field).
			if (rank_str !== undefined && rank_str !== "")
			{
				const rank = Number(rank_str);
				if (!Number.isNaN(rank) && rank < this._log_min_level) { return false; }
			}
		}
		if (this._log_filter && !line.textContent.toLowerCase().includes(this._log_filter))
		{
			return false;
		}
		return true;
	}

	_apply_log_filter()
	{
		if (!this._log_body)
		{
			return;
		}
		for (const line of this._log_body.querySelectorAll(".sessions-log-line"))
		{
			line.style.display = this._line_passes_filters(line) ? "" : "none";
		}
	}
}