fix(maestro): keep sidebar accent on the picked session while previewing

A preview moved the active tab's focus, and the sidebar took that as the
focused session, so hovering another row dropped the accent from the row the
operator had actually opened. Report the pre-preview focus instead.

web/src/multiview.ts

@@ -215,6 +215,20 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     return fl?.sessionId ?? null;
   }
 
+  /** The session the operator actually picked — what `focusedLeafSession()`
+   *  would return with the preview undone. A preview moves the active tab and
+   *  its focus, but it is a view: the accent stays on the row the operator
+   *  chose until they choose another (docs/PREVIEW.md). */
+  function realFocusedSession(): string | null {
+    const p = preview;
+    if (!p) return focusedLeafSession();
+    const tab = layout.tabs.find((t) => t.id === p.prevActiveTabId) ?? activeTab(layout);
+    const leafId =
+      p.prevFocus && p.prevFocus.tabId === tab.id ? p.prevFocus.leafId : tab.focusedLeafId;
+    const fl = leafId ? findLeaf(tab.root, leafId) : null;
+    return fl?.sessionId ?? null;
+  }
+
   /** Update focus styling + pool focus + sidebar highlights without rebuilding
    *  the DOM (cheap path for click-to-focus). */
   function syncFocus(): void {
@@ -233,7 +247,7 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     if (!preview) pool.setFocus(focusedLeafSession());
     sidebar.setHighlights({
       open: realOpenSessions(),
-      focused: focusedLeafSession(),
+      focused: realFocusedSession(),
     });
     // The row the preview came from wears the same colour, so the sidebar and
     // the pane read as one thing rather than two unrelated highlights.
@@ -608,7 +622,7 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
       { focus: !preview && !quietRender },
     );
     if (!preview) pool.setFocus(focused.sessionId ?? null);
-    sidebar.setHighlights({ open: realOpenSessions(), focused: focused.sessionId ?? null });
+    sidebar.setHighlights({ open: realOpenSessions(), focused: realFocusedSession() });
     sidebar.setPreview(preview?.sessionId ?? null);
   }
 
@@ -817,7 +831,7 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     openSessions: realOpenSessions,
     beginSessionDrag,
     render,
-    focusedSessionId: focusedLeafSession,
+    focusedSessionId: realFocusedSession,
     previewSession,
     previewedSessionId: () => preview?.sessionId ?? null,
   };

web/src/preview.test.ts

@@ -47,12 +47,20 @@ function fakePool(): Spy {
 
 /** Last value the controller pushed to `SidebarHandle.setPreview`. */
 const previewRow: { id: string | null } = { id: null };
+/** Last highlights the controller pushed to `SidebarHandle.setHighlights`. */
+const rowHighlights: { open: Set<string>; focused: string | null } = {
+  open: new Set(),
+  focused: null,
+};
 
 function fakeSidebar(): SidebarHandle {
   return {
     el: document.createElement("aside"),
     setActive() {},
-    setHighlights() {},
+    setHighlights(h) {
+      rowHighlights.open = h.open;
+      rowHighlights.focused = h.focused;
+    },
     setPreview(id: string | null) {
       previewRow.id = id;
     },
@@ -95,6 +103,8 @@ describe("sidebar preview", () => {
     stubFetch();
     document.body.replaceChildren();
     previewRow.id = null;
+    rowHighlights.open = new Set();
+    rowHighlights.focused = null;
   });
 
   afterEach(() => {
@@ -134,6 +144,28 @@ describe("sidebar preview", () => {
     expect(mv.el.querySelectorAll(".mv-pane.is-preview")).toHaveLength(0);
   });
 
+  it("leaves the accent on the session the pane actually holds", () => {
+    const layout = twoTabs();
+    const spy = fakePool();
+    const mv = makeMv(layout, spy.pool);
+    expect(rowHighlights.focused).toBe("s2");
+
+    // Walking past other rows never moves the accent off what the operator
+    // picked: only a real open does (docs/PREVIEW.md).
+    mv.previewSession("s1");
+    expect(rowHighlights.focused).toBe("s2");
+    expect(mv.focusedSessionId()).toBe("s2");
+
+    mv.previewSession("s9");
+    expect(rowHighlights.focused).toBe("s2");
+
+    mv.previewSession(null);
+    expect(rowHighlights.focused).toBe("s2");
+
+    mv.openSession("s1");
+    expect(rowHighlights.focused).toBe("s1");
+  });
+
   it("shows a session no pane holds in a throwaway tab that leaves no trace", () => {
     const layout = twoTabs();
     const before = serializeLayout(layout);