Keep TUI zero state stable during shell startup (#14632)

## Description
- Keep the empty TUI zero state vertically stable while the temporary
`Starting shell...` hint is visible.
- Account for the hint row and its top padding only in the bootstrap
zero-state slot; transcript, long-running-command, and GUI paths remain
unchanged.
- Add a presenter-based 80×40 regression covering bootstrap and ready
states.

Originating thread:
https://warpdotdev.slack.com/archives/C0BDQDW8V5E/p1785652323329829

## Linked Issue
- [x] The request is tracked in
[APP-5098](https://linear.app/warpdotdev/issue/APP-5098/tui-zero-state-shifts-when-starting-shell-disappears).
- [x] Real live-TUI `tmux capture-pane` proof is included below.

## Testing
- [x] `zero_state_position_stays_stable_across_shell_bootstrap` failed
before the fix and passes after it.
- [x] `bootstrap_renders_starting_shell_above_input` and
`long_running_command_keeps_input_hidden` pass.
- [x] `./script/format --check`
- [x] `cargo clippy -p warp_tui --all-targets --tests -- -D warnings`
- [x] `cargo nextest run -p warp_tui --lib` — 949/949 passed.
- [x] `cargo build -p warp_tui --bin warp-tui-oss`
- [x] `cargo build -p warp_tui --bin warp-tui-dev`

Full workspace presubmit completed format, inline-test, clippy,
clang-format, and WGSL checks, but its workspace-test phase exceeded the
sandbox target filesystem twice (`No space left on device`); the scoped
TUI suite and PR CI are fully green.

### Real live TUI proof
These frames came directly from `tmux capture-pane` while one
authenticated `warp-tui-dev` process ran in an 80×40 PTY. They are
terminal output from the real TUI, not replayed presenter buffers.

- [Bootstrap frame — full terminal
capture](https://staging.warp.dev/api/v1/agent/artifacts/019fc1a8-0abb-76b7-925e-2a8a669a16bf/download)
- [Ready frame from the same process — full terminal
capture](https://staging.warp.dev/api/v1/agent/artifacts/019fc1a8-0d3b-73db-9ee4-a32f78dc6ef5/download)

Relevant lines (one-based `capture-pane` output; equivalent zero-based
rows are 13/32/33):

```
bootstrap  14: Warp Agent CLI
bootstrap  33: Starting shell...
bootstrap  34: ▁▁▁...  (input top border)
ready      14: Warp Agent CLI
ready      34: ▁▁▁...  (input top border)
```

## Agent Mode
- [x] Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-TUI: Fixed the zero state shifting when shell startup
completes.

### Rework changes
- ⚠️ **Real TUI proof:** installed the pinned internal channel-config
generator, built the dev/dogfood TUI, authenticated with the inherited
cloud-runner API key, and captured bootstrap plus ready frames from the
same live tmux session. The title and input border remain on identical
rows while `Starting shell...` disappears.
- No code changes were needed; the review finding concerned verification
evidence only.
<!-- factory-agent:
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-->

_Conversation:
https://staging.warp.dev/conversation/5020fa91-cade-4b17-8d57-af8c5de70f0c_
_Run:
https://oz.staging.warp.dev/runs/019fc139-4de5-70c1-a4ef-4a772853163a_

Co-Authored-By: Oz <oz-agent@warp.dev>
_This PR was generated with [Oz](https://warp.dev/oz)._

Co-authored-by: Oz <oz-agent@warp.dev>

Author: warp-agent-staging[bot]

crates/warp_tui/src/terminal_session_view.rs

@@ -192,6 +192,9 @@ const RUNNING_COMMAND_DETACH_HINT: &str = "ctrl-c to return to command";
 /// Replaces the exit hint when viewing a child agent conversation.
 pub(crate) const CTRL_C_KILL_CHILD_HINT: &str = "ctrl-c again to kill child agent";
 const STARTING_SHELL_HINT: &str = "Starting shell...";
+/// The hint row plus its top padding. The zero state accounts for this temporary
+/// chrome so its centered position already matches the post-bootstrap layout.
+const STARTING_SHELL_CHROME_ROWS: u16 = 2;
 const SETTINGS_PARSE_FAILED_HINT: &str = "Settings failed to load: invalid syntax.";
 const SETTINGS_INVALID_VALUES_HINT: &str = "Settings failed to load: invalid values.";
 
@@ -5145,6 +5148,8 @@ impl TuiTerminalSessionView {
         let builder = TuiUiBuilder::from_app(ctx);
         let orchestration_tabs_available = state.orchestration_available();
         let blocker_active = state.has_blocking_interaction();
+        let show_starting_shell_hint =
+            !blocker_active && matches!(input_target, TuiInputTarget::Disabled);
 
         if state.is_alt_screen() {
             self.zero_state_interaction.set_visible(false);
@@ -5224,10 +5229,20 @@ impl TuiTerminalSessionView {
         let mut content = TuiFlex::column();
         let transcript_is_empty = self.transcript.as_ref(ctx).is_empty();
         self.zero_state_interaction.set_visible(transcript_is_empty);
-        if transcript_is_empty && self.session_state.as_ref(ctx).show_first_zero_state() {
-            content = content.flex_child(self.zero_state_view.as_ref(ctx).render_first_run(ctx));
-        } else if transcript_is_empty {
-            content = content.flex_child(TuiChildView::new(&self.zero_state_view).finish());
+        if transcript_is_empty {
+            let zero_state = if self.session_state.as_ref(ctx).show_first_zero_state() {
+                self.zero_state_view.as_ref(ctx).render_first_run(ctx)
+            } else {
+                TuiChildView::new(&self.zero_state_view).finish()
+            };
+            let zero_state = if show_starting_shell_hint {
+                TuiContainer::new(zero_state)
+                    .with_padding_top(STARTING_SHELL_CHROME_ROWS)
+                    .finish()
+            } else {
+                zero_state
+            };
+            content = content.flex_child(zero_state);
         } else {
             content = content.flex_child(TuiChildView::new(&self.transcript).finish());
         }
@@ -5239,7 +5254,7 @@ impl TuiTerminalSessionView {
         // fresh each pass — no stored suppression flag — and the hidden
         // input model is never written to, so its draft/cursor/selection/
         // scroll survive untouched.
-        if !blocker_active && matches!(input_target, TuiInputTarget::Disabled) {
+        if show_starting_shell_hint {
             content = content.child(
                 TuiContainer::new(
                     TuiText::new(STARTING_SHELL_HINT)

crates/warp_tui/src/terminal_session_view_tests.rs

@@ -2996,6 +2996,49 @@ fn bootstrap_renders_starting_shell_above_input() {
     });
 }
 
+#[test]
+fn zero_state_position_stays_stable_across_shell_bootstrap() {
+    App::test((), |mut app| async move {
+        let fixture = focus_test_fixture(&mut app);
+        let (view, _) = add_focus_test_session(&mut app, &fixture, true);
+
+        let ready_lines = render_session(&mut app, &view, 80, 40);
+        assert!(
+            ready_lines
+                .iter()
+                .all(|line| line.trim() != "Starting shell..."),
+            "ready state must not render the bootstrap hint:\n{}",
+            ready_lines.join("\n")
+        );
+
+        view.update(&mut app, |view, _| {
+            view.terminal_model.lock().block_list_mut().reinit_shell();
+        });
+        let bootstrap_lines = render_session(&mut app, &view, 80, 40);
+        assert!(
+            bootstrap_lines
+                .iter()
+                .any(|line| line.trim() == "Starting shell..."),
+            "bootstrap state must render the starting-shell hint:\n{}",
+            bootstrap_lines.join("\n")
+        );
+
+        let title_row = |lines: &[String]| {
+            lines
+                .iter()
+                .position(|line| line.contains("Warp Agent CLI"))
+                .unwrap_or_else(|| panic!("zero-state title should render:\n{}", lines.join("\n")))
+        };
+        assert_eq!(
+            title_row(&bootstrap_lines),
+            title_row(&ready_lines),
+            "zero state must not shift when the bootstrap hint disappears\nbootstrap:\n{}\nready:\n{}",
+            bootstrap_lines.join("\n"),
+            ready_lines.join("\n")
+        );
+    });
+}
+
 /// The input child's rendered element is cached by the presenter, and
 /// transcript emptiness can flip without any input-owned event (a terminal
 /// block landing via the PTY wakeup path only invalidates the session view).