fix(web): sync canvaskit size before first paint

crates/op-host-web/src/canvaskit.rs

@@ -796,7 +796,11 @@ pub async fn mount_ck(canvas_id: String) -> Result<(), JsValue> {
         canvas: canvas.clone(),
         a11y,
     }));
-    inner.borrow_mut().repaint();
+    {
+        let mut b = inner.borrow_mut();
+        let _ = b.resize_to_window(&window)?;
+        b.repaint();
+    }
     crate::web_fonts::drain_font_requests(&inner);
 
     // Populate the chat model picker from the daemon's `/api/ai/models`

crates/op-host-web/tests/web_mount_event_wiring.rs

@@ -28,6 +28,25 @@ fn canvaskit_mount_listens_for_window_resize() {
     );
 }
 
+#[test]
+fn canvaskit_mount_syncs_window_size_before_first_repaint() {
+    let source = std::fs::read_to_string(concat!(env!("CARGO_MANIFEST_DIR"), "/src/canvaskit.rs"))
+        .expect("canvaskit source is readable");
+    let start = source
+        .find("let inner = Rc::new(RefCell::new(CkInner")
+        .expect("mount creates CkInner");
+    let first_repaint = source[start..]
+        .find("repaint();")
+        .map(|idx| start + idx)
+        .expect("mount performs an initial repaint");
+    let initial_mount = &source[start..first_repaint];
+
+    assert!(
+        initial_mount.contains("resize_to_window(&window)"),
+        "CanvasKit mount must sync the backend to the current browser viewport before the first repaint"
+    );
+}
+
 #[test]
 fn canvaskit_mount_does_not_panic_if_a11y_mirror_borrow_is_busy() {
     let source = std::fs::read_to_string(concat!(env!("CARGO_MANIFEST_DIR"), "/src/canvaskit.rs"))