Bug 1529992 - disable the MergeClipLeaf optimization for all shadows properly. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D27114

--HG--
extra : moz-landing-system : lando

gfx/layers/wr/ClipManager.cpp

@@ -155,16 +155,10 @@ wr::WrSpaceAndClipChain ClipManager::SwitchItem(nsDisplayItem* aItem) {
   // some overhead further down the pipeline.
   bool separateLeaf = false;
   if (clip && clip->mASR == asr && clip->mClip.GetRoundedRectCount() == 0) {
-    if (type == DisplayItemType::TYPE_TEXT) {
-      // Text with shadows interprets the text display item clip rect and
-      // clips from the clip chain differently.
-      separateLeaf = !aItem->Frame()->StyleText()->HasTextShadow();
-    } else {
-      // Container display items are not currently supported because the clip
-      // rect of a stacking context is not handled the same as normal display
-      // items.
-      separateLeaf = aItem->GetChildren() == nullptr;
-    }
+    // Container display items are not currently supported because the clip
+    // rect of a stacking context is not handled the same as normal display
+    // items.
+    separateLeaf = aItem->GetChildren() == nullptr;
   }
 
   ItemClips clips(asr, clip, separateLeaf);

gfx/webrender_bindings/WebRenderAPI.cpp

@@ -1094,11 +1094,48 @@ void DisplayListBuilder::PushShadow(const wr::LayoutRect& aRect,
                                     const wr::LayoutRect& aClip,
                                     bool aIsBackfaceVisible,
                                     const wr::Shadow& aShadow) {
-  wr_dp_push_shadow(mWrState, aRect, MergeClipLeaf(aClip), aIsBackfaceVisible,
+  // Local clip_rects are translated inside of shadows, as they are assumed to
+  // be part of the element drawing itself, and not a parent frame clipping it.
+  // As such, it is not sound to apply the MergeClipLeaf optimization inside of
+  // shadows. So we disable the optimization when we encounter a shadow.
+  // Shadows don't span frames, so we don't have to worry about MergeClipLeaf
+  // being re-enabled mid-shadow. The optimization is restored in PopAllShadows.
+  SuspendClipLeafMerging();
+  wr_dp_push_shadow(mWrState, aRect, aClip, aIsBackfaceVisible,
                     &mCurrentSpaceAndClipChain, aShadow);
 }
 
-void DisplayListBuilder::PopAllShadows() { wr_dp_pop_all_shadows(mWrState); }
+void DisplayListBuilder::PopAllShadows() {
+  wr_dp_pop_all_shadows(mWrState);
+  ResumeClipLeafMerging();
+}
+
+void DisplayListBuilder::SuspendClipLeafMerging() {
+  if (mClipChainLeaf) {
+    // No one should reinitialize mClipChainLeaf while we're suspended
+    MOZ_ASSERT(!mSuspendedClipChainLeaf);
+
+    mSuspendedClipChainLeaf = mClipChainLeaf;
+    mSuspendedSpaceAndClipChain = Some(mCurrentSpaceAndClipChain);
+
+    // Clip is implicitly parented by mCurrentSpaceAndClipChain
+    auto clipId = DefineClip(Nothing(), *mClipChainLeaf);
+    auto clipChainId = DefineClipChain({ clipId });
+
+    mCurrentSpaceAndClipChain.clip_chain = clipChainId.id;
+    mClipChainLeaf = Nothing();
+  }
+}
+
+void DisplayListBuilder::ResumeClipLeafMerging() {
+  if (mSuspendedClipChainLeaf) {
+    mCurrentSpaceAndClipChain = *mSuspendedSpaceAndClipChain;
+    mClipChainLeaf = mSuspendedClipChainLeaf;
+
+    mSuspendedClipChainLeaf = Nothing();
+    mSuspendedSpaceAndClipChain = Nothing();
+  }
+}
 
 void DisplayListBuilder::PushBoxShadow(
     const wr::LayoutRect& aRect, const wr::LayoutRect& aClip,

gfx/webrender_bindings/WebRenderAPI.h

@@ -591,6 +591,10 @@ class DisplayListBuilder final {
     return aClip;
   }
 
+  // See the implementation of PushShadow for details on these methods.
+  void SuspendClipLeafMerging();
+  void ResumeClipLeafMerging();
+
   wr::WrState* mWrState;
 
   // Track each scroll id that we encountered. We use this structure to
@@ -606,6 +610,11 @@ class DisplayListBuilder final {
   // there is no clip rect to merge with.
   Maybe<wr::LayoutRect> mClipChainLeaf;
 
+  // Versions of the above that are on hold while SuspendClipLeafMerging is on
+  // (see the implementation of PushShadow for details).
+  Maybe<wr::WrSpaceAndClipChain> mSuspendedSpaceAndClipChain;
+  Maybe<wr::LayoutRect> mSuspendedClipChainLeaf;
+
   RefPtr<layout::TextDrawTarget> mCachedTextDT;
   RefPtr<gfxContext> mCachedContext;