Bug 1298209 - Make clip capturing easier to understand. r=mattwoodrow

MozReview-Commit-ID: 6OmdH7WoFma

--HG--
extra : rebase_source : 9fc53828b07034b26c6a6075d34bb4c2c4cb1110

layout/generic/nsFrame.cpp

@@ -2271,10 +2271,59 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   nsDisplayListBuilder::AutoBuildingDisplayList
     buildingDisplayList(aBuilder, this, dirtyRect, true);
 
+  // Depending on the effects that are applied to this frame, we can create
+  // multiple container display items and wrap them around our contents.
+  // This enum lists all the potential container display items, in the order
+  // outside to inside.
+  enum class ContainerItemType : uint8_t {
+    eNone = 0,
+    eOwnLayerIfNeeded,
+    eBlendMode,
+    eFixedPosition,
+    eStickyPosition,
+    eOwnLayerForTransformWithRoundedClip,
+    ePerspective,
+    eTransform,
+    eSeparatorTransforms,
+    eOpacity,
+    eSVGEffects,
+    eBlendContainer
+  };
+
   DisplayListClipState::AutoSaveRestore clipState(aBuilder);
 
+  // If there is a current clip, then depending on the container items we
+  // create, different things can happen to it. Some container items simply
+  // propagate the clip to their children and aren't clipped themselves.
+  // But other container items, especially those that establish a different
+  // geometry for their contents (e.g. transforms), capture the clip on
+  // themselves and unset the clip for their contents. If we create more than
+  // one of those container items, the clip will be captured on the outermost
+  // one and the inner container items will be unclipped.
+  ContainerItemType clipCapturedBy = ContainerItemType::eNone;
+  if (useFixedPosition) {
+    clipCapturedBy = ContainerItemType::eFixedPosition;
+  } else if (useStickyPosition) {
+    clipCapturedBy = ContainerItemType::eStickyPosition;
+  } else if (isTransformed) {
+    if ((hasPerspective || extend3DContext) && clipState.SavedStateHasRoundedCorners()) {
+      // If we're creating an nsDisplayTransform item that is going to combine
+      // its transform with its children (preserve-3d or perspective), then we
+      // can't have an intermediate surface. Mask layers force an intermediate
+      // surface, so if we're going to need both then create a separate
+      // wrapping layer for the mask.
+      clipCapturedBy = ContainerItemType::eOwnLayerForTransformWithRoundedClip;
+    } else if (hasPerspective) {
+      clipCapturedBy = ContainerItemType::ePerspective;
+    } else {
+      clipCapturedBy = ContainerItemType::eTransform;
+    }
+  } else if (usingSVGEffects) {
+    clipCapturedBy = ContainerItemType::eSVGEffects;
+  }
+
   bool clearClip = false;
-  if (isTransformed || usingSVGEffects || useFixedPosition || useStickyPosition) {
+  if (clipCapturedBy != ContainerItemType::eNone) {
     // We don't need to pass ancestor clipping down to our children;
     // everything goes inside a display item's child list, and the display
     // item itself will be clipped.
@@ -2411,18 +2460,15 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
                                                      containerItemScrollClip));
   }
 
-  if (!isTransformed && !useFixedPosition && !useStickyPosition) {
-    // Restore saved clip state now so that any display items we create below
-    // are clipped properly.
-    clipState.ExitStackingContextContents(&containerItemScrollClip);
-  }
-
   /* If there are any SVG effects, wrap the list up in an SVG effects item
    * (which also handles CSS group opacity). Note that we create an SVG effects
    * item even if resultList is empty, since a filter can produce graphical
    * output even if the element being filtered wouldn't otherwise do so.
    */
   if (usingSVGEffects) {
+    if (clipCapturedBy == ContainerItemType::eSVGEffects) {
+      clipState.ExitStackingContextContents(&containerItemScrollClip);
+    }
     // Revert to the post-filter dirty rect.
     buildingDisplayList.SetDirtyRect(dirtyRectOutsideSVGEffects);
     /* List now emptied, so add the new list to the top. */
@@ -2488,19 +2534,9 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
     resultList.AppendToTop(&participants);
   }
 
-  /* If we're creating an nsDisplayTransform item that is going to combine its transform
-   * with its children (preserve-3d or perspective), then we can't have an intermediate
-   * surface. Mask layers force an intermediate surface, so if we're going to need both
-   * then create a separate wrapping layer for the mask.
-   */
-  bool needsLayerForMask = isTransformed && (extend3DContext || hasPerspective) &&
-                           clipState.SavedStateHasRoundedCorners();
-  NS_ASSERTION(!Combines3DTransformWithAncestors() || !clipState.SavedStateHasRoundedCorners(),
-               "Can't support mask layers on intermediate preserve-3d frames");
-
   if (isTransformed && !resultList.IsEmpty()) {
-    // Restore clip state now so nsDisplayTransform is clipped properly.
+    if (clipCapturedBy == ContainerItemType::eTransform) {
-    if (!hasPerspective && !useFixedPosition && !useStickyPosition && !needsLayerForMask) {
+      // Restore clip state now so nsDisplayTransform is clipped properly.
       clipState.ExitStackingContextContents(&containerItemScrollClip);
     }
     // Revert to the dirtyrect coming in from the parent, without our transform
@@ -2529,7 +2565,7 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
     }
 
     if (hasPerspective) {
-      if (!useFixedPosition && !useStickyPosition && !needsLayerForMask) {
+      if (clipCapturedBy == ContainerItemType::ePerspective) {
         clipState.ExitStackingContextContents(&containerItemScrollClip);
       }
       resultList.AppendNewToTop(
@@ -2539,11 +2575,8 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
     }
   }
 
-  if (useFixedPosition || useStickyPosition || needsLayerForMask) {
+  if (clipCapturedBy == ContainerItemType::eOwnLayerForTransformWithRoundedClip) {
     clipState.ExitStackingContextContents(&containerItemScrollClip);
-  }
-
-  if (needsLayerForMask) {
     resultList.AppendNewToTop(
       new (aBuilder) nsDisplayOwnLayer(aBuilder, this, &resultList, 0,
                                        mozilla::layers::FrameMetrics::NULL_SCROLL_ID,
@@ -2553,9 +2586,15 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   /* If we have sticky positioning, wrap it in a sticky position item.
    */
   if (useFixedPosition) {
+    if (clipCapturedBy == ContainerItemType::eFixedPosition) {
+      clipState.ExitStackingContextContents(&containerItemScrollClip);
+    }
     resultList.AppendNewToTop(
         new (aBuilder) nsDisplayFixedPosition(aBuilder, this, &resultList));
   } else if (useStickyPosition) {
+    if (clipCapturedBy == ContainerItemType::eStickyPosition) {
+      clipState.ExitStackingContextContents(&containerItemScrollClip);
+    }
     resultList.AppendNewToTop(
         new (aBuilder) nsDisplayStickyPosition(aBuilder, this, &resultList));
   }