perf(sidecar): skip redundant paint barriers

crates/ely_servo_host/src/bin/ely_servo_sidecar/live.rs

@@ -201,12 +201,13 @@ fn poll_frame(
 ) -> Result<LiveOutcome, LiveSidecarError> {
     host.tick();
     let snapshot = host.snapshot(&session.webview_id)?;
-    if !should_paint_live_frame(snapshot.has_pending_frame(), session.awaiting_visible_frame) {
+    let has_pending_frame = snapshot.has_pending_frame();
+    if !should_paint_live_frame(has_pending_frame, session.awaiting_visible_frame) {
         return Ok(LiveOutcome::empty());
     }
 
     let (outcome, has_visible_content) =
-        paint_pending_frame(host, session, rendering_context_kind)?;
+        paint_pending_frame(host, session, rendering_context_kind, has_pending_frame)?;
     if has_visible_content {
         session.awaiting_visible_frame = false;
         session.ever_visible_frame = true;
@@ -227,22 +228,26 @@ fn paint_pending_frame(
     host: &mut SoftwareServoHost,
     session: &mut LiveSession,
     rendering_context_kind: RenderingContextKind,
+    has_pending_frame: bool,
 ) -> Result<(LiveOutcome, bool), LiveSidecarError> {
     match rendering_context_kind {
-        RenderingContextKind::Software => paint_readback_frame(host, session),
+        RenderingContextKind::Software => paint_readback_frame(host, session, !has_pending_frame),
         #[cfg(all(feature = "hardware-render", target_os = "macos"))]
-        RenderingContextKind::Hardware => paint_hardware_surface_frame(host, session),
+        RenderingContextKind::Hardware => {
+            paint_hardware_surface_frame(host, session, has_pending_frame)
+        }
         #[cfg(not(all(feature = "hardware-render", target_os = "macos")))]
-        RenderingContextKind::Hardware => paint_readback_frame(host, session),
+        RenderingContextKind::Hardware => paint_readback_frame(host, session, !has_pending_frame),
     }
 }
 
 fn paint_readback_frame(
     host: &mut SoftwareServoHost,
     session: &LiveSession,
+    wait_for_completion: bool,
 ) -> Result<(LiveOutcome, bool), LiveSidecarError> {
     let paint_started_at = Instant::now();
-    host.paint(&session.webview_id)?;
+    host.paint_with_readback(&session.webview_id, wait_for_completion)?;
     let snapshot = host.snapshot(&session.webview_id)?;
     let frame = host.last_rendered_frame()?;
     let paint_ns = elapsed_ns(paint_started_at);
@@ -259,24 +264,26 @@ fn paint_readback_frame(
 fn paint_hardware_surface_frame(
     host: &mut SoftwareServoHost,
     session: &LiveSession,
+    has_pending_frame: bool,
 ) -> Result<(LiveOutcome, bool), LiveSidecarError> {
     if !session.ever_visible_frame {
-        return paint_initial_hardware_surface_frame(host, session);
+        return paint_initial_hardware_surface_frame(host, session, !has_pending_frame);
     }
     // Cross-process IOSurface lookup can block the app-side worker for
     // seconds on macOS. Live app frames use readback so scroll/click
     // input stays bounded by the paint barrier instead of the surface
     // import path.
-    paint_readback_frame(host, session)
+    paint_readback_frame(host, session, !has_pending_frame)
 }
 
 #[cfg(all(feature = "hardware-render", target_os = "macos"))]
 fn paint_initial_hardware_surface_frame(
     host: &mut SoftwareServoHost,
     session: &LiveSession,
+    wait_for_completion: bool,
 ) -> Result<(LiveOutcome, bool), LiveSidecarError> {
     let paint_started_at = Instant::now();
-    host.paint(&session.webview_id)?;
+    host.paint_with_readback(&session.webview_id, wait_for_completion)?;
     let snapshot = host.snapshot(&session.webview_id)?;
     let frame = host.last_rendered_frame()?;
     let paint_ns = elapsed_ns(paint_started_at);

crates/ely_servo_host/src/runtime.rs

@@ -111,7 +111,7 @@ impl SoftwareServoHost {
     /// bytes. The live hardware path exports the just-presented
     /// IOSurface from the rendering context.
     pub fn paint_without_readback(&mut self, webview_id: &WebViewId) -> Result<(), ServoHostError> {
-        self.paint_webview(webview_id, false).map(|_| ())
+        self.paint_webview(webview_id, false, true).map(|_| ())
     }
 
     pub fn close_webview(&mut self, webview_id: &WebViewId) -> bool {
@@ -146,6 +146,7 @@ impl SoftwareServoHost {
         &mut self,
         webview_id: &WebViewId,
         capture_frame: bool,
+        wait_for_completion: bool,
     ) -> Result<Option<RenderedFrame>, ServoHostError> {
         let rendering_context = self.webview(webview_id)?.rendering_context.clone();
         rendering_context.make_current().map_err(|_| ServoHostError::RenderingContextNotCurrent)?;
@@ -154,18 +155,20 @@ impl SoftwareServoHost {
         // thread — it does NOT block until the framebuffer is consistent.
         // Without a barrier, `read_rendered_frame` below races the paint
         // thread and reliably reads the cleared-white state on data: URLs.
-        // Clear the pending-frame flag first so we can detect the *next*
-        // `notify_new_frame_ready` (the one our `paint()` triggers), then
-        // pump the event loop until Servo reports the new frame is ready
-        // or `paint_barrier_budget()` elapses. On timeout we fall through
-        // and read anyway, preserving the pre-T15 fast path for callers
-        // that explicitly disable the barrier with `ELY_PAINT_BARRIER_MS=0`.
+        // Clear the pending-frame flag first so barrier callers can
+        // detect the *next* `notify_new_frame_ready` (the one our
+        // `paint()` triggers), then pump the event loop until Servo
+        // reports the new frame is ready or `paint_barrier_budget()`
+        // elapses. When the caller already observed a pending frame,
+        // readback can use that ready frame and skip the extra wait.
         {
             let webview = self.webview(webview_id)?;
             webview.delegate.mark_frame_presented();
             webview.webview.paint();
         }
+        if wait_for_completion {
             self.wait_for_paint_completion(webview_id);
+        }
         let rendered_frame = if capture_frame {
             Some(Self::read_rendered_frame(rendering_context.as_ref())?)
         } else {
@@ -175,6 +178,19 @@ impl SoftwareServoHost {
         self.webview(webview_id)?.delegate.mark_frame_presented();
         Ok(rendered_frame)
     }
+
+    pub fn paint_with_readback(
+        &mut self,
+        webview_id: &WebViewId,
+        wait_for_completion: bool,
+    ) -> Result<(), ServoHostError> {
+        let Some(rendered_frame) = self.paint_webview(webview_id, true, wait_for_completion)?
+        else {
+            return Err(ServoHostError::RenderedFrameUnavailable);
+        };
+        self.last_rendered_frame = Some(rendered_frame);
+        Ok(())
+    }
 }
 
 impl ServoHost for SoftwareServoHost {
@@ -383,11 +399,7 @@ impl ServoHost for SoftwareServoHost {
     }
 
     fn paint(&mut self, webview_id: &WebViewId) -> Result<(), ServoHostError> {
-        let Some(rendered_frame) = self.paint_webview(webview_id, true)? else {
-            return Err(ServoHostError::RenderedFrameUnavailable);
-        };
-        self.last_rendered_frame = Some(rendered_frame);
-        Ok(())
+        self.paint_with_readback(webview_id, true)
     }
 
     fn last_rendered_frame(&self) -> Result<RenderedFrame, ServoHostError> {