Bug 1954798 - Add debug markers in WR's display list. r=emilio

And use DebugMarker(1) to signify that the next item is a view-transition snapshot.
This specific marker is not meant to stay forever (although the infrastructure is), but it is useful right now.

Differential Revision: https://phabricator.services.mozilla.com/D242023

gfx/webrender_bindings/WebRenderAPI.cpp

@@ -1783,6 +1783,10 @@ void DisplayListBuilder::PushBoxShadow(
                         aBorderRadius, aClipMode);
 }
 
+void DisplayListBuilder::PushDebug(uint32_t aVal) {
+  wr_dp_push_debug(mWrState, aVal);
+}
+
 void DisplayListBuilder::StartGroup(nsPaintedDisplayItem* aItem) {
   if (!mDisplayItemCache || mDisplayItemCache->IsFull()) {
     return;

gfx/webrender_bindings/WebRenderAPI.h

@@ -793,6 +793,8 @@ class DisplayListBuilder final {
                      const wr::BorderRadius& aBorderRadius,
                      const wr::BoxShadowClipMode& aClipMode);
 
+  void PushDebug(uint32_t aVal);
+
   /**
    * Notifies the DisplayListBuilder that it can group together WR display items
    * that are pushed until |CancelGroup()| or |FinishGroup()| call.

gfx/webrender_bindings/src/bindings.rs

@@ -3039,6 +3039,11 @@ pub extern "C" fn wr_dp_push_stacking_context(
     result
 }
 
+#[no_mangle]
+pub extern "C" fn wr_dp_push_debug(state: &mut WrState, val: u32) {
+    state.frame_builder.dl_builder.push_debug(val);
+}
+
 #[no_mangle]
 pub extern "C" fn wr_dp_pop_stacking_context(state: &mut WrState, is_reference_frame: bool) {
     debug_assert!(unsafe { !is_in_render_thread() });

gfx/wr/webrender/src/scene_building.rs

@@ -1949,6 +1949,7 @@ impl<'a> SceneBuilder<'a> {
 
                 self.pop_all_shadows();
             }
+            DisplayItem::DebugMarker(..) => {}
         }
     }
 

gfx/wr/webrender_api/src/display_item.rs

@@ -205,6 +205,9 @@ pub enum DisplayItem {
 
     ReuseItems(ItemKey),
     RetainedItems(ItemKey),
+
+    // For debugging purposes.
+    DebugMarker(u32),
 }
 
 /// This is a "complete" version of the DisplayItem, with all implicit trailing
@@ -247,6 +250,8 @@ pub enum DebugDisplayItem {
     PopReferenceFrame,
     PopStackingContext,
     PopAllShadows,
+
+    DebugMarker(u32)
 }
 
 #[derive(Clone, Copy, Debug, Default, Deserialize, PartialEq, Serialize, PeekPoke)]
@@ -2294,6 +2299,7 @@ impl DisplayItem {
             DisplayItem::Text(..) => "text",
             DisplayItem::YuvImage(..) => "yuv_image",
             DisplayItem::BackdropFilter(..) => "backdrop_filter",
+            DisplayItem::DebugMarker(..) => "debug",
         }
     }
 }

gfx/wr/webrender_api/src/display_list.rs

@@ -709,6 +709,7 @@ impl BuiltDisplayList {
                 Real::PopAllShadows => Debug::PopAllShadows,
                 Real::ReuseItems(_) |
                 Real::RetainedItems(_) => unreachable!("Unexpected item"),
+                Real::DebugMarker(val) => Debug::DebugMarker(val),
             };
             debug_items.push(serial_di);
         }
@@ -1921,6 +1922,10 @@ impl DisplayListBuilder {
         }
     }
 
+    pub fn push_debug(&mut self, val: u32) {
+        self.push_item(&di::DisplayItem::DebugMarker(val));
+    }
+
     fn generate_clip_index(&mut self) -> di::ClipId {
         self.next_clip_index += 1;
         di::ClipId(self.next_clip_index - 1, self.pipeline_id)

layout/generic/nsImageFrame.cpp

@@ -2374,6 +2374,7 @@ void nsDisplayImage::MaybeCreateWebRenderCommandsForViewTransition(
   const auto destRect =
       wr::ToLayoutRect(LayoutDeviceRect::FromAppUnits(destAppUnits, factor));
   auto rendering = wr::ToImageRendering(frame->UsedImageRendering());
+  aBuilder.PushDebug(1);
   aBuilder.PushImage(destRect, destRect, !BackfaceIsHidden(),
                      /* aForceAntiAliasing = */ false, rendering, key);
 }