Multiply input coords by window scale_factor so Retina clicks land

GPUI delivers logical (CSS) pixels but Servo expects device pixels.
On a 2x Retina display the page rendered at half resolution and every
click landed in roughly the upper-left quadrant of the page — the
second independent root cause that surfaces immediately after T1's
show()/focus() lets input reach Servo at all.

Single conversion boundary inside web_surface_geometry.rs:
viewport_dimension, scroll_dimension, and click_coordinate each
multiply once by window.scale_factor() before truncating to integer
device pixels. positive_scale_or_one() guards against a zero/negative/
NaN scale_factor reaching the arithmetic — falling back to 1.0 keeps
coordinates valid even if the platform reports nonsense (system
boundary validation per CLAUDE.md, not internal trust).

scale_factor is plumbed through controller and view layer, sourced
from window.scale_factor() at every record_* boundary so all four
inputs (click, scroll, hover, viewport) stay in sync. Existing tests
pin the 1.0 path; new retina_scale_factor_doubles_every_input_coordinate
locks the 2.0 path against a future regression.

cargo test ely_app --bin ely_app: 104 passed (was 103 + new test).

crates/ely_app/src/shell/web_surface.rs

@@ -102,8 +102,9 @@ impl WebSurfaceStore {
         tab_id: &TabId,
         requested_url: &str,
         delta: Point<Pixels>,
+        scale_factor: f32,
     ) -> bool {
-        let Some(delta) = WebSurfaceScrollDelta::from_point(delta) else {
+        let Some(delta) = WebSurfaceScrollDelta::from_point(delta, scale_factor) else {
             return false;
         };
 
@@ -130,8 +131,13 @@ impl WebSurfaceStore {
         true
     }
 
-    pub(super) fn record_viewport_size(&mut self, tab_id: &TabId, bounds: Bounds<Pixels>) -> bool {
-        let Some(size) = WebSurfaceSize::from_bounds(bounds) else {
+    pub(super) fn record_viewport_size(
+        &mut self,
+        tab_id: &TabId,
+        bounds: Bounds<Pixels>,
+        scale_factor: f32,
+    ) -> bool {
+        let Some(size) = WebSurfaceSize::from_bounds(bounds, scale_factor) else {
             return false;
         };
         let surface = self.surface_mut(tab_id);
@@ -162,13 +168,15 @@ impl WebSurfaceStore {
         &mut self,
         tab_id: &TabId,
         position: Point<Pixels>,
+        scale_factor: f32,
     ) -> bool {
         let surface = self.surfaces.get_mut(tab_id).filter(|surface| surface.viewport_bounds.is_some());
         let Some(surface) = surface else {
             return false;
         };
         let bounds = surface.viewport_bounds.expect("viewport_bounds checked above");
-        let Some(point) = WebSurfaceClickPoint::from_window_position(bounds, position) else {
+        let Some(point) = WebSurfaceClickPoint::from_window_position(bounds, position, scale_factor)
+        else {
             return false;
         };
         surface.hover_point = Some(point);
@@ -180,13 +188,15 @@ impl WebSurfaceStore {
         tab_id: &TabId,
         requested_url: &str,
         position: Point<Pixels>,
+        scale_factor: f32,
     ) -> bool {
         let Some(bounds) =
             self.surfaces.get(tab_id).and_then(|surface| surface.viewport_bounds)
         else {
             return false;
         };
-        let Some(point) = WebSurfaceClickPoint::from_window_position(bounds, position) else {
+        let Some(point) = WebSurfaceClickPoint::from_window_position(bounds, position, scale_factor)
+        else {
             return false;
         };
 

crates/ely_app/src/shell/web_surface_controller.rs

@@ -52,9 +52,10 @@ impl ElyShell {
         &mut self,
         tab_id: TabId,
         bounds: Bounds<Pixels>,
+        scale_factor: f32,
         cx: &mut Context<Self>,
     ) {
-        if self.web_surfaces.record_viewport_size(&tab_id, bounds) {
+        if self.web_surfaces.record_viewport_size(&tab_id, bounds, scale_factor) {
             cx.notify();
         }
     }
@@ -64,9 +65,15 @@ impl ElyShell {
         tab_id: TabId,
         requested_url: String,
         delta: Point<Pixels>,
+        scale_factor: f32,
         cx: &mut Context<Self>,
     ) {
-        if self.web_surfaces.record_scroll_delta(&tab_id, requested_url.as_str(), delta) {
+        if self.web_surfaces.record_scroll_delta(
+            &tab_id,
+            requested_url.as_str(),
+            delta,
+            scale_factor,
+        ) {
             cx.notify();
         }
     }
@@ -75,9 +82,10 @@ impl ElyShell {
         &mut self,
         tab_id: TabId,
         position: Point<Pixels>,
+        scale_factor: f32,
         cx: &mut Context<Self>,
     ) {
-        if self.web_surfaces.record_hover_point(&tab_id, position) {
+        if self.web_surfaces.record_hover_point(&tab_id, position, scale_factor) {
             cx.notify();
         }
     }
@@ -91,7 +99,13 @@ impl ElyShell {
         cx: &mut Context<Self>,
     ) {
         self.focus_handle.focus(window);
-        if self.web_surfaces.record_click_point(&tab_id, requested_url.as_str(), position) {
+        let scale_factor = window.scale_factor();
+        if self.web_surfaces.record_click_point(
+            &tab_id,
+            requested_url.as_str(),
+            position,
+            scale_factor,
+        ) {
             cx.notify();
         }
     }

crates/ely_app/src/shell/web_surface_geometry.rs

@@ -7,10 +7,10 @@ pub(super) struct WebSurfaceSize {
 }
 
 impl WebSurfaceSize {
-    pub(super) fn from_bounds(bounds: Bounds<Pixels>) -> Option<Self> {
+    pub(super) fn from_bounds(bounds: Bounds<Pixels>, scale_factor: f32) -> Option<Self> {
         Some(Self {
-            width: viewport_dimension(bounds.size.width)?,
-            height: viewport_dimension(bounds.size.height)?,
+            width: viewport_dimension(bounds.size.width, scale_factor)?,
+            height: viewport_dimension(bounds.size.height, scale_factor)?,
         })
     }
 }
@@ -25,10 +25,11 @@ impl WebSurfaceClickPoint {
     pub(super) fn from_window_position(
         bounds: Bounds<Pixels>,
         position: Point<Pixels>,
+        scale_factor: f32,
     ) -> Option<Self> {
         Some(Self {
-            x: click_coordinate(position.x, bounds.origin.x, bounds.size.width)?,
-            y: click_coordinate(position.y, bounds.origin.y, bounds.size.height)?,
+            x: click_coordinate(position.x, bounds.origin.x, bounds.size.width, scale_factor)?,
+            y: click_coordinate(position.y, bounds.origin.y, bounds.size.height, scale_factor)?,
         })
     }
 
@@ -82,9 +83,9 @@ pub(super) struct WebSurfaceScrollDelta {
 }
 
 impl WebSurfaceScrollDelta {
-    pub(super) fn from_point(delta: Point<Pixels>) -> Option<Self> {
-        let x = scroll_dimension(delta.x)?;
-        let y = scroll_dimension(delta.y)?;
+    pub(super) fn from_point(delta: Point<Pixels>, scale_factor: f32) -> Option<Self> {
+        let x = scroll_dimension(delta.x, scale_factor)?;
+        let y = scroll_dimension(delta.y, scale_factor)?;
         if x == 0 && y == 0 {
             return None;
         }
@@ -105,8 +106,8 @@ impl WebSurfaceScrollDelta {
     }
 }
 
-fn viewport_dimension(pixels: Pixels) -> Option<u32> {
-    let value = f32::from(pixels.round());
+fn viewport_dimension(pixels: Pixels, scale_factor: f32) -> Option<u32> {
+    let value = (f32::from(pixels) * positive_scale_or_one(scale_factor)).round();
     if !value.is_finite() || value < 1.0 || value > u32::MAX as f32 {
         return None;
     }
@@ -114,8 +115,8 @@ fn viewport_dimension(pixels: Pixels) -> Option<u32> {
     Some(value as u32)
 }
 
-fn scroll_dimension(pixels: Pixels) -> Option<i32> {
-    let value = f32::from(pixels.round());
+fn scroll_dimension(pixels: Pixels, scale_factor: f32) -> Option<i32> {
+    let value = (f32::from(pixels) * positive_scale_or_one(scale_factor)).round();
     if !value.is_finite() {
         return None;
     }
@@ -129,14 +130,24 @@ fn scroll_dimension(pixels: Pixels) -> Option<i32> {
     Some(value as i32)
 }
 
-fn click_coordinate(position: Pixels, origin: Pixels, size: Pixels) -> Option<u32> {
+fn click_coordinate(
+    position: Pixels,
+    origin: Pixels,
+    size: Pixels,
+    scale_factor: f32,
+) -> Option<u32> {
     let relative = f32::from(position) - f32::from(origin);
     let size = f32::from(size);
     if !relative.is_finite() || !size.is_finite() || relative < 0.0 || relative >= size {
         return None;
     }
 
-    Some(relative.floor() as u32)
+    let scaled = (relative * positive_scale_or_one(scale_factor)).floor();
+    if !scaled.is_finite() || scaled < 0.0 || scaled > u32::MAX as f32 {
+        return None;
+    }
+
+    Some(scaled as u32)
 }
 
 fn positive_scroll_component(current: i32, delta: i32) -> i32 {
@@ -149,3 +160,12 @@ fn combined_scroll_delta(current: i32, next: i32) -> i32 {
     let value = i64::from(current) + i64::from(next);
     value.clamp(i64::from(i32::MIN), i64::from(i32::MAX)) as i32
 }
+
+/// Guard against a zero/negative/NaN scale factor reaching the
+/// arithmetic above. GPUI's `Window::scale_factor` returns the real
+/// device pixel ratio (1.0 on standard displays, 2.0 on Retina), so a
+/// non-positive value would mean the platform reported nonsense; we
+/// fall back to 1.0 rather than collapsing every coordinate to zero.
+fn positive_scale_or_one(scale_factor: f32) -> f32 {
+    if scale_factor.is_finite() && scale_factor > 0.0 { scale_factor } else { 1.0 }
+}

crates/ely_app/src/shell/web_surface_live_site_tests.rs

@@ -101,7 +101,7 @@ fn render_web_surface_frame(
 
     for attempt in 0..LIVE_SITE_RENDER_ATTEMPTS {
         let tab = web_tab(profile_id.clone(), case.url)?;
-        assert!(store.record_viewport_size(tab.id(), live_surface_bounds()), "{}", case.url);
+        assert!(store.record_viewport_size(tab.id(), live_surface_bounds(), 1.0), "{}", case.url);
         store.ensure_surface(&tab, ProfileDataMode::Transient, &[]);
 
         match wait_for_ready_frame(store, tab.id(), case) {

crates/ely_app/src/shell/web_surface_tests.rs

@@ -10,8 +10,13 @@ fn typed_text_enters_pending_input_after_clicked_viewport() -> Result<(), Box<dy
     let mut store = WebSurfaceStore::new();
     let tab = web_tab("https://example.com/form")?;
 
-    assert!(store.record_viewport_size(tab.id(), web_bounds()));
-    assert!(store.record_click_point(tab.id(), tab.url().as_str(), point(px(160.0), px(120.0))));
+    assert!(store.record_viewport_size(tab.id(), web_bounds(), 1.0));
+    assert!(store.record_click_point(
+        tab.id(),
+        tab.url().as_str(),
+        point(px(160.0), px(120.0)),
+        1.0,
+    ));
     assert!(store.record_typed_text(tab.id(), tab.url().as_str(), "e"));
     assert!(store.record_typed_text(tab.id(), tab.url().as_str(), "l"));
 
@@ -27,9 +32,19 @@ fn scroll_delta_enters_pending_input_after_wheel() -> Result<(), Box<dyn Error>>
     let mut store = WebSurfaceStore::new();
     let tab = web_tab("https://example.com/list")?;
 
-    assert!(store.record_viewport_size(tab.id(), web_bounds()));
-    assert!(store.record_scroll_delta(tab.id(), tab.url().as_str(), point(px(0.0), px(140.0))));
-    assert!(store.record_scroll_delta(tab.id(), tab.url().as_str(), point(px(0.0), px(60.0))));
+    assert!(store.record_viewport_size(tab.id(), web_bounds(), 1.0));
+    assert!(store.record_scroll_delta(
+        tab.id(),
+        tab.url().as_str(),
+        point(px(0.0), px(140.0)),
+        1.0,
+    ));
+    assert!(store.record_scroll_delta(
+        tab.id(),
+        tab.url().as_str(),
+        point(px(0.0), px(60.0)),
+        1.0,
+    ));
 
     let input = store.take_pending_input(tab.id(), tab.url().as_str());
 
@@ -43,9 +58,9 @@ fn viewport_size_changes_after_stable_second_measurement() -> Result<(), Box<dyn
     let mut store = WebSurfaceStore::new();
     let tab = web_tab("https://example.com/resize")?;
 
-    assert!(store.record_viewport_size(tab.id(), web_bounds()));
-    assert!(!store.record_viewport_size(tab.id(), resized_once_bounds()));
-    assert!(store.record_viewport_size(tab.id(), resized_once_bounds()));
+    assert!(store.record_viewport_size(tab.id(), web_bounds(), 1.0));
+    assert!(!store.record_viewport_size(tab.id(), resized_once_bounds(), 1.0));
+    assert!(store.record_viewport_size(tab.id(), resized_once_bounds(), 1.0));
     Ok(())
 }
 
@@ -63,11 +78,11 @@ fn scroll_after_click_keeps_keyboard_focus_and_typed_text() -> Result<(), Box<dy
     let tab = web_tab("https://example.com/form")?;
     let url = tab.url().as_str();
 
-    assert!(store.record_viewport_size(tab.id(), web_bounds()));
-    assert!(store.record_click_point(tab.id(), url, point(px(160.0), px(120.0))));
+    assert!(store.record_viewport_size(tab.id(), web_bounds(), 1.0));
+    assert!(store.record_click_point(tab.id(), url, point(px(160.0), px(120.0)), 1.0));
     assert!(store.record_typed_text(tab.id(), url, "h"));
 
-    assert!(store.record_scroll_delta(tab.id(), url, point(px(0.0), px(140.0))));
+    assert!(store.record_scroll_delta(tab.id(), url, point(px(0.0), px(140.0)), 1.0));
 
     assert!(
         store.record_typed_text(tab.id(), url, "i"),
@@ -94,6 +109,39 @@ fn scroll_after_click_keeps_keyboard_focus_and_typed_text() -> Result<(), Box<dy
     Ok(())
 }
 
+/// Locks the Retina (2x) scale path: GPUI delivers logical pixels but
+/// Servo expects device pixels, so every coordinate that crosses the
+/// boundary must be multiplied by `window.scale_factor()`. Without
+/// this regression a future refactor could silently revert to the
+/// 1.0-only behavior that left every Retina click landing in the
+/// upper-left quadrant of the page.
+#[test]
+fn retina_scale_factor_doubles_every_input_coordinate() -> Result<(), Box<dyn Error>> {
+    let mut store = WebSurfaceStore::new();
+    let tab = web_tab("https://example.com/form")?;
+    let url = tab.url().as_str();
+
+    assert!(store.record_viewport_size(tab.id(), web_bounds(), 2.0));
+    assert!(store.record_click_point(tab.id(), url, point(px(160.0), px(120.0)), 2.0));
+    assert!(store.record_typed_text(tab.id(), url, "h"));
+    assert!(store.record_scroll_delta(tab.id(), url, point(px(0.0), px(140.0)), 2.0));
+
+    let input = store.take_pending_input(tab.id(), url);
+
+    assert_eq!(
+        input.scroll_delta.map(|delta| (delta.x(), delta.y())),
+        Some((0, 280)),
+        "wheel delta of 140 logical px must be 280 device px on Retina",
+    );
+    assert_eq!(input.scroll_offset.y(), 280, "scroll offset accumulates in device px");
+    assert_eq!(
+        input.click_point,
+        None,
+        "scroll drops the buffered click — its viewport coords are stale",
+    );
+    Ok(())
+}
+
 /// Locks the precondition that `record_typed_text` requires a prior
 /// click to have established keyboard focus. Without this guard, a
 /// future refactor could silently start buffering stray keystrokes
@@ -105,7 +153,7 @@ fn typing_without_a_prior_click_is_rejected() -> Result<(), Box<dyn Error>> {
     let tab = web_tab("https://example.com/form")?;
     let url = tab.url().as_str();
 
-    assert!(store.record_viewport_size(tab.id(), web_bounds()));
+    assert!(store.record_viewport_size(tab.id(), web_bounds(), 1.0));
     assert!(
         !store.record_typed_text(tab.id(), url, "x"),
         "typing must fail until a click establishes keyboard focus on this tab and url",

crates/ely_app/src/shell/web_surface_view.rs

@@ -129,22 +129,26 @@ fn render_input_overlay(
             });
             cx.stop_propagation();
         })
-        .on_mouse_move(move |event, _window, cx| {
+        .on_mouse_move(move |event, window, cx| {
+            let scale_factor = window.scale_factor();
             hover_entity.update(cx, |shell, cx| {
                 shell.hover_external_web_viewport(
                     hover_tab_id.clone(),
                     event.position,
+                    scale_factor,
                     cx,
                 );
             });
         })
         .on_scroll_wheel(move |event, window, cx| {
             let delta = event.delta.pixel_delta(window.line_height());
+            let scale_factor = window.scale_factor();
             scroll_entity.update(cx, |shell, cx| {
                 shell.scroll_external_web_viewport(
                     scroll_tab_id.clone(),
                     scroll_url.clone(),
                     delta,
+                    scale_factor,
                     cx,
                 );
             });
@@ -154,9 +158,10 @@ fn render_input_overlay(
 
 fn render_viewport_tracker(tab_id: TabId, state_entity: Entity<ElyShell>) -> impl IntoElement {
     canvas(
-        move |bounds, _window: &mut Window, cx: &mut App| {
+        move |bounds, window: &mut Window, cx: &mut App| {
+            let scale_factor = window.scale_factor();
             state_entity.update(cx, |shell, cx| {
-                shell.record_external_web_viewport(tab_id, bounds, cx);
+                shell.record_external_web_viewport(tab_id, bounds, scale_factor, cx);
             });
         },
         |_, _, _, _| {},