feat(p2): sidebar branch rows behind the session's expand control

Branch rows are hidden until the session row's expand control (the
subagents' chevron) opens them, above the subagents; the control also
appears for a session with two or more branches and no subagents, with
the branch count beside the subagent count. Showing a subagent still
expands the list; showing any other non-main conversation no longer
does (right after a /tree switch the page shows the new branch before
the row names it main).

apps/p2/docs/tree.md

@@ -115,7 +115,9 @@ node:
 - Web: "Edit" beside the owner's view of a user message (on hover; always shown on touch screens) and "Branch from
   here" under an answer, in the main conversation. Under the session in the sidebar, from two branches on, one row per
   branch (`Sessions` rows' `branches`: the stored labels, null for the session title, newest first in a stable order,
-  the current one marked in place); choosing one switches the session to its end (`SessionTree.select`). The worker
+  the current one marked in place), behind the session row's expand control, which also shows its subagents
+  (collapsed by default, expanded while a subagent is shown); choosing one switches the session to its end
+  (`SessionTree.select`). The worker
   reports `p2.main` (`treeSelf`: `main` and the branches with their labels) at start and whenever a commit changes it.
 - Search indexes each branch's own entries (after its fork point).
 

apps/p2/src/web/Drawer.tsx

@@ -71,8 +71,8 @@ function ThemeButton() {
 }
 
 /**
- * A session's `/tree` branches (docs/tree.md), from two on, newest first in a stable order: switching only moves the
- * "current" mark. Choosing another branch switches the session to it (`onBranch`, the owner's); the pencil beside a
+ * A session's `/tree` branches (docs/tree.md), newest first in a stable order: switching only moves the "current" mark.
+ * Shown from two branches on, behind the session row's expand control (`hidden` while collapsed). Choosing another branch switches the session to it (`onBranch`, the owner's); the pencil beside a
  * row renames it (`onRenameBranch`). A branch without a label (conversation 1's default) shows the session title.
  */
 function BranchRows({
@@ -82,6 +82,7 @@ function BranchRows({
 	onOpen,
 	onBranch,
 	onRenameBranch,
+	hidden,
 }: {
 	session: SessionSummary;
 	conversationId: number;
@@ -89,10 +90,10 @@ function BranchRows({
 	onOpen: (id: string, conversationId?: number) => void;
 	onBranch: (id: string, conversationId: number) => void;
 	onRenameBranch?: (id: string, conversationId: number) => void;
+	hidden: boolean;
 }) {
-	if (session.branches.length < 2) return null;
 	return (
-		<div className="branch-rows">
+		<div className="branch-rows" id={`branches-${session.id}`} hidden={hidden}>
 			{session.branches.map((branch) => {
 				const current = branch.conversation === session.main;
 				const label = branch.label ?? session.title;
@@ -151,11 +152,13 @@ function SessionRow({
 	archived?: boolean;
 	children: ReactNode;
 }) {
-	// A subagent shown expands its session's list.
-	const [expanded, setExpanded] = useState(conversationId !== session.main);
+	// One control expands a session's branch rows and its subagents; a subagent shown expands them. Not any conversation
+	// but the main one: right after a `/tree` switch the page shows the new branch before the row names it main.
+	const showsSubagent = session.subagents.some((agent) => agent.conversationId === conversationId);
+	const [expanded, setExpanded] = useState(showsSubagent);
 	useEffect(() => {
-		if (conversationId !== session.main) setExpanded(true);
-	}, [conversationId, session.main]);
+		if (showsSubagent) setExpanded(true);
+	}, [showsSubagent]);
 	// One-shot agents are opened from their script's tool card; archived subagents wait behind their own row.
 	const listed = (archived ? [] : session.subagents)
 		.filter((agent) => agent.kind !== "agent")
@@ -164,6 +167,8 @@ function SessionRow({
 	const stored = listed.filter((agent) => agent.archived);
 	const [showStored, setShowStored] = useState(false);
 	const storedOpen = showStored || stored.some((agent) => agent.conversationId === conversationId);
+	const branches = onBranch !== undefined && !archived && session.branches.length >= 2;
+	const nested = [branches && "branches", listed.length > 0 && "subagents"].filter(Boolean).join(" and ");
 	const row = (agent: (typeof listed)[number]) => (
 		<button
 			type="button"
@@ -190,25 +195,33 @@ function SessionRow({
 	);
 	return (
 		<div className="session-row">
-			<div className={`session-header ${listed.length ? "has-subagents" : ""}`}>
+			<div className={`session-header ${nested ? "has-subagents" : ""}`}>
 				{children}
-				{listed.length > 0 && (
+				{nested && (
 					<button
 						type="button"
 						className="subagent-toggle"
-						aria-label={expanded ? "Collapse subagents" : "Expand subagents"}
+						aria-label={`${expanded ? "Collapse" : "Expand"} ${nested}`}
 						aria-expanded={expanded}
-						aria-controls={`subagents-${session.id}`}
+						aria-controls={[branches && `branches-${session.id}`, listed.length > 0 && `subagents-${session.id}`]
+							.filter(Boolean)
+							.join(" ")}
 						onClick={() => setExpanded(!expanded)}
 					>
-						<span>{agents.length}</span>
+						{listed.length > 0 && <span>{agents.length}</span>}
+						{branches && (
+							<span className="toggle-branches" title={`${session.branches.length} branches`}>
+								<BranchIcon size={11} />
+								{session.branches.length}
+							</span>
+						)}
 						<span className={`chev ${expanded ? "open" : ""}`}>
 							<ChevronIcon size={12} />
 						</span>
 					</button>
 				)}
 			</div>
-			{onBranch && !archived && (
+			{branches && (
 				<BranchRows
 					session={session}
 					conversationId={conversationId}
@@ -216,6 +229,7 @@ function SessionRow({
 					onOpen={onOpen}
 					onBranch={onBranch}
 					onRenameBranch={onRenameBranch}
+					hidden={!expanded}
 				/>
 			)}
 			{listed.length > 0 && (

apps/p2/src/web/style.css

@@ -2845,6 +2845,12 @@ button.diff-number:hover {
 .subagent-toggle:hover {
 	color: var(--text);
 }
+/* The branch count beside the subagent count: the fork icon before it. */
+.toggle-branches {
+	display: inline-flex;
+	align-items: center;
+	gap: 2px;
+}
 .subagent-row {
 	min-height: var(--control-sm);
 	padding-top: 2px;

apps/p2/test/p2/ui-drawer.test.ts

@@ -17,6 +17,7 @@ import { Catalog, Sessions } from "../../plugins/sessions/contract.ts";
 import { Sharing } from "../../plugins/sharing/contract.ts";
 import { SessionStatus } from "../../plugins/status/contract.ts";
 import { Subagents } from "../../plugins/subagents/contract.ts";
+import { SessionTree } from "../../plugins/tree/contract.ts";
 import { Browser, browserTests } from "./browser.ts";
 import { type Fixture, holdIn, poll, startFixture, until } from "./helpers.ts";
 
@@ -184,6 +185,29 @@ describe("drawer and new-session parity", { skip: !browserTests }, () => {
 		const status = await holdIn(owner, SessionStatus, {}, "reader");
 		await childChat.stop(BACKGROUND_CONTEXT);
 		await until(status.statuses, (states) => !states?.[pinned.id]?.busy.includes(child.conversationId));
+		// With `/tree` branches as well, the one control shows the branch rows, then the subagents, and hides both.
+		await until(status.statuses, (states) => states?.[pinned.id]?.tasks === 0);
+		const tree = await holdIn(owner, SessionTree, { session: pinned.id }, "owner");
+		const first = (await tree.tree(BACKGROUND_CONTEXT)).nodes.find((node) => node.role === "user")!;
+		await tree.navigate({ entry: first.entry }, BACKGROUND_CONTEXT);
+		await tree.select({ conversation: 1 }, BACKGROUND_CONTEXT);
+		tree.release();
+		const toggle = `document.querySelector('.subagent-toggle[aria-controls="branches-${pinned.id} subagents-${pinned.id}"]')`;
+		await browser.wait(`${toggle}?.getAttribute('aria-label')==='Collapse branches and subagents'`);
+		assert.deepEqual(
+			await browser.evaluate<string[]>(
+				`[...document.getElementById('branches-${pinned.id}').parentElement.children].slice(1).map(e=>e.id+(e.hidden?':hidden':''))`,
+			),
+			[`branches-${pinned.id}`, `subagents-${pinned.id}`],
+		);
+		await browser.evaluate(`${toggle}.click()`);
+		await browser.wait(
+			`document.getElementById('branches-${pinned.id}').hidden && document.getElementById('subagents-${pinned.id}').hidden`,
+		);
+		assert.equal(
+			await browser.evaluate<string>(`${toggle}.getAttribute('aria-label')`),
+			"Expand branches and subagents",
+		);
 	});
 
 	it("preserves rename, pin and archive after moving actions out of session rows", async () => {

apps/p2/test/p2/ui-tree.test.ts

@@ -45,8 +45,9 @@ describe("branches in the browser", { skip: !browserTests }, () => {
 		await browser.metrics(1280, 900);
 		await browser.open(`http://127.0.0.1:${fixture.server.port}/s/${id}`);
 		await browser.wait(`document.querySelector('.messages')?.textContent.includes('Echo: second')`);
-		// One branch: no branch rows yet.
+		// One branch: no branch rows and no expand control yet.
 		assert.equal(await browser.evaluate<number>(`document.querySelectorAll('.branch-row').length`), 0);
+		assert.equal(await browser.evaluate<number>(`document.querySelectorAll('.subagent-toggle').length`), 0);
 		await browser.evaluate(
 			`[...document.querySelectorAll('.user')].find(e=>e.textContent.includes('second')).querySelector('.edit-button').click()`,
 		);
@@ -58,8 +59,16 @@ describe("branches in the browser", { skip: !browserTests }, () => {
 		await browser.wait(
 			`document.querySelector('.messages')?.textContent.includes('Echo: first') && !document.querySelector('.messages').textContent.includes('Echo: second')`,
 		);
-		// The fork has a row at once, newest first, labelled with the edited message; conversation 1 shows the title.
+		// The fork has a row at once, newest first, labelled with the edited message; conversation 1 shows the title. The
+		// rows wait collapsed behind the session's expand control (here without subagents), which shows them.
 		await browser.wait(`document.querySelectorAll('.branch-row').length===2`);
+		const branches = `document.getElementById('branches-${id}')`;
+		const toggle = `document.querySelector('.subagent-toggle[aria-controls="branches-${id}"]')`;
+		assert.equal(await browser.evaluate<boolean>(`${branches}.hidden`), true);
+		assert.equal(await browser.evaluate<string>(`${toggle}.getAttribute('aria-label')`), "Expand branches");
+		assert.equal(await browser.evaluate<string>(`${toggle}.textContent`), "2");
+		await browser.evaluate(`${toggle}.click()`);
+		await browser.wait(`!${branches}.hidden && ${toggle}.getAttribute('aria-expanded')==='true'`);
 		assert.deepEqual(
 			await browser.evaluate<string[]>(`[...document.querySelectorAll('.branch-row')].map(e=>e.textContent)`),
 			["secondcurrent", "Branches"],
@@ -72,10 +81,16 @@ describe("branches in the browser", { skip: !browserTests }, () => {
 		await browser.wait(`location.pathname==='/s/${id}'`);
 		await browser.wait(`document.querySelector('.messages')?.textContent.includes('Echo: second')`);
 		assert.equal((await row()).main, 1);
-		// The rows keep their order: only the "current" mark moves.
+		// The rows keep their order: only the "current" mark moves. They stay expanded across the switch.
 		await browser.wait(
 			`[...document.querySelectorAll('.branch-row')].map(e=>e.textContent).join('|')==='second|Branchescurrent'`,
 		);
+		assert.equal(await browser.evaluate<boolean>(`${branches}.hidden`), false);
+		// Collapsing hides them again.
+		await browser.evaluate(`${toggle}.click()`);
+		await browser.wait(`${branches}.hidden && ${toggle}.getAttribute('aria-label')==='Expand branches'`);
+		await browser.evaluate(`${toggle}.click()`);
+		await browser.wait(`!${branches}.hidden`);
 	});
 
 	it("renames a branch from its row; conversation 1 follows the session title until renamed", async () => {