Lift the web canvas out of in-flow so the input overlay lands on screen

A GPUI harness boots a real ElyShell, navigates to an external URL, and
asks the input_overlay's sibling canvas tracker where it laid out. On
main before this change the canvas reports

    Bounds { origin: (309, window_height - 17), size: (W - 326, content_h) }

i.e. the overlay's top edge sits at the very bottom of the visible
window. Every user click in the visible area lands above (or beside)
the overlay; the on_mouse_down + capture_any_mouse_up listeners never
even see the event because the hitbox is off-screen. Twelve commits
chased focus/coords/outcome enums on the sidecar side while every click
in the live shell hit empty space.

Root cause: in render_web_surface the rendered web image (img / loading
div / error page) was a non-absolute child of a `.relative().size_full()`
wrapper. The non-absolute child claims `size_full` block-flow height
inside that wrapper, which made the wrapper's intrinsic height
content_height + content_height. The two `.absolute().size_full()`
siblings (viewport_tracker, input_overlay) then sized against that
inflated parent and were positioned in the bottom half — exactly
content_height below where they were supposed to be.

Fix: keep the relative wrapper as the layout owner of the panel slot
(size_full, overflow_hidden, min_w_0) and put the rendered image into
an absolute `inset_0` child of its own. viewport_tracker and
input_overlay stay as absolute siblings. With the image out of in-flow
the wrapper sizes to its parent and the overlay's hitbox lands at
y = top of content area (71 in a 1080-tall window) instead of
y = window_height - 17.

GPUI test harness (`gpui_harness_tests.rs`) is the holdout set:
  - `baseline_overlay_div_receives_simulated_click` proves GPUI's
    occlude + capture_any_mouse_up primitive works under TestAppContext.
  - `baseline_overlay_with_full_listener_combo_receives_click` proves
    the exact listener combo render_input_overlay uses works in
    isolation.
  - `ely_shell_external_canvas_lays_out_inside_window` boots a real
    ElyShell, navigates, and asserts the overlay's measured bounds fit
    inside the visible window. Without the fix above, this test trips
    on bounds extending below the window bottom.

The three new store-layer tests in web_surface_tests.rs pin per-tab
isolation, zero-delta short-circuit, and resize-mid-drain decoupling
invariants the harness work flushed out.

ely_app picks up gpui's test-support feature as a dev-dependency so the
harness can use VisualTestContext + simulate_mouse_*.

cargo test --bin ely_app: 112 passed (was 108 + 4 new harness/store tests).

Remaining work (not in this commit): even with the layout fixed, the
harness shows MouseUp's capture_any_mouse_up still doesn't fire on the
ElyShell tree, while MouseDown's bubble does. Some sibling/ancestor
listener in the live shell is eating the MouseUp capture phase that
the standalone listener-combo baseline does not. Tracked separately.

crates/ely_app/src/shell/gpui_harness_tests.rs

@@ -0,0 +1,197 @@
+//! GPUI test harness for the input pipeline.
+//!
+//! Twelve sidecar-side commits and one shell-side commit had all claimed to
+//! fix "click does nothing" while the user kept reporting the same symptom.
+//! The roundtable consensus: every store-layer test passed GREEN, every
+//! sidecar integration test passed GREEN, but nothing in the repo exercised
+//! the real GPUI event tree (`render_input_overlay` + window-level mouse
+//! handlers + sidebar capture interactions). This module is that missing
+//! holdout set.
+//!
+//! What we have so far:
+//!   1. `baseline_overlay_div_receives_simulated_click` — proves GPUI's
+//!      `.occlude()` + `capture_any_mouse_up` primitive works correctly in
+//!      `TestAppContext`. If this regresses, the harness itself is broken.
+//!   2. `baseline_overlay_with_full_listener_combo_receives_click` — proves
+//!      the exact listener combo `render_input_overlay` uses (on_mouse_down +
+//!      capture_any_mouse_up + on_mouse_move + on_scroll_wheel on a single
+//!      `.occlude()` div) works in isolation.
+//!   3. `ely_shell_external_canvas_lays_out_inside_window` — boots a real
+//!      `ElyShell`, navigates to an external URL, and asserts the
+//!      input_overlay's measured viewport bounds fit inside the visible
+//!      window. This was the original ship-blocker: the overlay was being
+//!      positioned at `y = window_height - 17`, entirely below the visible
+//!      region, so every user click hit empty space above the overlay.
+
+use std::cell::RefCell;
+use std::rc::Rc;
+
+use ely_domain::{TabId, UrlText};
+use gpui::{
+    Bounds, Context, IntoElement, Modifiers, MouseButton, ParentElement, Pixels, Render,
+    Styled, TestAppContext, Window, div, point, px,
+};
+use gpui::InteractiveElement;
+
+use super::ShellState;
+use super::web_surface::WebSurfaceStore;
+
+#[cfg(test)]
+impl super::ElyShell {
+    pub(super) fn web_surfaces_for_test(&self) -> &WebSurfaceStore {
+        &self.web_surfaces
+    }
+}
+
+#[gpui::test]
+async fn ely_shell_external_canvas_lays_out_inside_window(cx: &mut TestAppContext) {
+    cx.update(|cx| gpui_component::init(cx));
+
+    let (shell, cx) = cx.add_window_view(|window, cx| super::ElyShell::new(window, cx));
+    cx.run_until_parked();
+
+    cx.update(|window, app_cx| {
+        shell.update(app_cx, |shell, ctx| {
+            shell.navigate_active_tab(
+                UrlText::parse("https://example.com/".to_string()).expect("valid URL"),
+                window,
+                ctx,
+            );
+        });
+    });
+    cx.run_until_parked();
+
+    let (_active_tab_id, active_tab_url, viewport_bounds) = active_tab_overlay_state(&shell, cx);
+    assert!(
+        active_tab_url.starts_with("https://"),
+        "active tab URL must be external https for render_external_web_canvas \
+         to render the input_overlay (got {active_tab_url:?})."
+    );
+    let bounds = viewport_bounds.unwrap_or_else(|| {
+        panic!(
+            "viewport_bounds for the active tab is None. The canvas tracker \
+             in render_input_overlay's sibling never fired its layout \
+             callback — render_external_web_canvas was not reached."
+        )
+    });
+    let window_size = cx.update(|window, _| window.bounds().size);
+    assert!(
+        bounds.origin.y + bounds.size.height <= window_size.height + px(1.0)
+            && bounds.origin.x + bounds.size.width <= window_size.width + px(1.0),
+        "Layout regression: input_overlay viewport_bounds {bounds:?} extend \
+         outside the {window_size:?} window. The canvas tracker measured a \
+         layout that escapes the visible region — every user click in the \
+         visible area now lands above (or beside) the overlay's hitbox. \
+         The original ship-blocker was bounds.origin.y == window_height-17 \
+         caused by content (the rendered web image) being a non-absolute \
+         child of the relative wrapper, which doubled the parent's height \
+         and pushed the overlay off-screen."
+    );
+}
+
+#[gpui::test]
+async fn baseline_overlay_with_full_listener_combo_receives_click(
+    cx: &mut TestAppContext,
+) {
+    let click_count = Rc::new(RefCell::new(0u32));
+    let counter_for_render = click_count.clone();
+
+    struct ComboProbe {
+        on_up_counter: Rc<RefCell<u32>>,
+    }
+    impl Render for ComboProbe {
+        fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+            let counter = self.on_up_counter.clone();
+            div().relative().size_full().child(
+                div()
+                    .absolute()
+                    .size_full()
+                    .occlude()
+                    .on_mouse_down(MouseButton::Left, |_event, _window, _cx| {})
+                    .capture_any_mouse_up(move |_event, _window, _cx| {
+                        *counter.borrow_mut() += 1;
+                    })
+                    .on_mouse_move(|_event, _window, _cx| {})
+                    .on_scroll_wheel(|_event, _window, _cx| {}),
+            )
+        }
+    }
+
+    let (_probe, cx) = cx.add_window_view(|_window, _cx| ComboProbe {
+        on_up_counter: counter_for_render,
+    });
+    cx.run_until_parked();
+
+    cx.simulate_mouse_move(point(px(100.0), px(100.0)), None, Modifiers::default());
+    cx.simulate_click(point(px(100.0), px(100.0)), Modifiers::default());
+    cx.run_until_parked();
+
+    assert_eq!(
+        *click_count.borrow(),
+        1,
+        "GPUI baseline with input_overlay's full listener combo: click should \
+         reach capture_any_mouse_up. If this fails, the listener combo itself \
+         is the problem, not the surrounding shell."
+    );
+}
+
+#[gpui::test]
+async fn baseline_overlay_div_receives_simulated_click(cx: &mut TestAppContext) {
+    let click_count = Rc::new(RefCell::new(0u32));
+    let counter_for_render = click_count.clone();
+
+    struct Probe {
+        on_up_counter: Rc<RefCell<u32>>,
+    }
+    impl Render for Probe {
+        fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+            let counter = self.on_up_counter.clone();
+            div().relative().size_full().child(
+                div().absolute().size_full().occlude().capture_any_mouse_up(
+                    move |_event, _window, _cx| {
+                        *counter.borrow_mut() += 1;
+                    },
+                ),
+            )
+        }
+    }
+
+    let (_probe, cx) = cx.add_window_view(|_window, _cx| Probe {
+        on_up_counter: counter_for_render,
+    });
+    cx.run_until_parked();
+
+    cx.simulate_mouse_move(point(px(100.0), px(100.0)), None, Modifiers::default());
+    cx.simulate_click(point(px(100.0), px(100.0)), Modifiers::default());
+    cx.run_until_parked();
+
+    assert_eq!(
+        *click_count.borrow(),
+        1,
+        "GPUI baseline: a div with .absolute().size_full().occlude() and \
+         capture_any_mouse_up never received the simulated click. The test \
+         harness or GPUI primitive is broken — ElyShell test results are \
+         meaningless until this passes."
+    );
+}
+
+fn active_tab_overlay_state(
+    shell: &gpui::Entity<super::ElyShell>,
+    cx: &mut gpui::VisualTestContext,
+) -> (TabId, String, Option<Bounds<Pixels>>) {
+    shell.read_with(cx, |shell, _cx| {
+        let tab = match &shell.state {
+            ShellState::Ready(core) => core.active_tab().expect("active tab exists"),
+            ShellState::StartupError(message) => {
+                panic!("ElyShell failed to start in test: {message}")
+            }
+        };
+        let tab_id = tab.id().clone();
+        let url = tab.url().as_str().to_string();
+        let bounds = shell
+            .web_surfaces_for_test()
+            .surface_for_test(&tab_id)
+            .and_then(|surface| surface.viewport_bounds);
+        (tab_id, url, bounds)
+    })
+}