test(terminal): follow the existing suites to the one geometry owner

The gate caught fourteen failures the focused tests could not: every harness
that builds a partial app out of cherry-picked mixin methods, and every source
guard that named `fitAddon.fit()` by hand.

Most are wiring — `syncTerminalGeometry`, `_refitAfterCellSizeChange` and
`_resizeTerminalTo` added to the fakes so the real chain runs rather than a
stub of it. `file-browser-search` is the one that shows why it matters: without
the method on the fake, selectSession's unconditional call threw into its own
catch and every later assertion in the file measured a load that never
happened.

Two are not wiring.

`detached-session-pane-sizing` pinned the behaviour this change deliberately
reverses. It asserted the LOCAL fit still runs for a session owned by its own
window — "withhold the send, never the reflow" — so the assertion is restated
rather than patched, with the reason beside it and in the file's docblock: a
reflow the PTY is never told about leaves this xterm rendering a CLI's frames
against a shape that does not exist, and the popup that owns the pane is
drawing for its own width regardless. The old rule bought a garbled frame, not
a correct one.

`mobile-prompt-composer` sliced `_cleanupSessionData` as a fixed 1200-character
window, so the assertion depended on how much unrelated code sat above the line
it cared about. It reads the whole method now.

`terminal-scroll-intent` records `syncTerminalGeometry` rather than `fit`,
under its own name: recording a bare fit there would name the very thing the
subject was changed to stop doing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/detached-session-pane-sizing.test.ts

@@ -14,6 +14,12 @@
  * already stood aside on the same condition, so this follows a rule the code
  * had already established.
  *
+ * ⚠️ It returns early BEFORE the local fit, not after (issue #464). The earlier
+ * rule was "withhold the send, never the reflow", which leaves this window's
+ * xterm at a shape the PTY was never told about — and a CLI computes its
+ * repaints from the shape it was told, so that reflow bought a garbled frame
+ * rather than a correct one. Withhold both, or neither.
+ *
  * Loaded via `vm` with a stubbed context (no jsdom — jsdom is broken on this
  * box; see connection-indicator.test.ts), the same way terminal-buffer-flush
  * extracts the real mixin methods from terminal-ui.js.
@@ -56,7 +62,13 @@ function makeApp(overrides: Record<string, unknown> = {}) {
   currentFetch = fetchMock;
   const app = {
     sendResize: mixin.sendResize,
+    // The real chain: sendResize fits, floors and applies through one function
+    // now, so the harness must let it (#464).
+    syncTerminalGeometry: mixin.syncTerminalGeometry,
+    _resizeTerminalTo: mixin._resizeTerminalTo,
+    _onPtyGeometryReport: vi.fn(),
     getTerminalDimensions: () => ({ cols: 120, rows: 40 }),
+    terminal: { cols: 120, rows: 40, resize: vi.fn() },
     fitAddon: { fit: vi.fn() },
     detachedSessions: new Set<string>(),
     isSoloWindow: false,
@@ -77,10 +89,16 @@ describe('detached sessions own their pane size', () => {
     expect(changed).toBe(false);
     // No request: the popup's size stands on the server.
     expect(fetchMock).not.toHaveBeenCalled();
-    // The LOCAL fit still runs, so the dashboard's own xterm stays correct and
-    // tab-rail-resize's single settle-time refit is not swallowed. Same line the
-    // mobile-keyboard guard draws: withhold the send, never the reflow.
-    expect((app.fitAddon as { fit: ReturnType<typeof vi.fn> }).fit).toHaveBeenCalled();
+    // ⚠️ REVERSED by issue #464, deliberately. This used to assert that the
+    // LOCAL fit still ran — "withhold the send, never the reflow" — on the
+    // reasoning that it keeps the dashboard's own xterm correct. It does not:
+    // it leaves this xterm at a shape the PTY was never told about, and Claude
+    // Code computes every repaint from the shape it WAS told, so the frames
+    // land on rows nothing erased. The popup that owns the PTY is drawing for
+    // its own width either way, so the dashboard's reflow was a reflow nothing
+    // was rendering for. Withholding the resize means withholding all of it.
+    expect((app.fitAddon as { fit: ReturnType<typeof vi.fn> }).fit).not.toHaveBeenCalled();
+    expect((app.terminal as { resize: ReturnType<typeof vi.fn> }).resize).not.toHaveBeenCalled();
   });
 
   it('the solo window still sizes the session it displays', async () => {