Bug 1343147 - Part 1. Do not double applying transform vector of the root frame in a glyph mask into the target context. r=mstange

When we generate the glyph mask for a transformed frame in
GenerateAndPushTextMask, the transform vector had been applied into aContext[1],
so we should find a way to prevent applying the vector again when painting the
glyph mask.

In bug 1299715, I tried to prevent double apply at [2], it caused two problems:
1. We only skip generating nsDisplayTransform, but we may still create a
nsDisplayPerspactive bellow. Since the parent of a nsDisplayPerspective must be
a nsDisplayTransform, which have been ignored, so we hit this assertion.
2. We skip all transform for all frames while painting the glyph mask, which is
not correct. We should only skip double applying transform vector of the root
frame.

This patch fixes both of these issues:
a. We will still create a nsDisplayTransform for the root frame if need. But
the transform matrix we apply into the target context will be an identity
matrix, so we fix #1 above.
b. In #a, we change the transform matrix to an identity matrix only for the root
frame of the  glyph mask, so we fix #2.

[1]
https://hg.mozilla.org/mozilla-central/file/59e5ec5729db/layout/painting/nsDisplayList.cpp#l752
[2]
https://hg.mozilla.org/mozilla-central/file/ce2c129f0a87/layout/generic/nsFrame.cpp#l2806

MozReview-Commit-ID: 973lkQQxLB6

--HG--
extra : rebase_source : aef80444e94d3af7eca776c981f8faded03bc985

layout/generic/nsFrame.cpp

@@ -2803,14 +2803,11 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
     buildingDisplayList.SetReferenceFrameAndCurrentOffset(outerReferenceFrame,
       GetOffsetToCrossDoc(outerReferenceFrame));
 
-    if (!aBuilder->IsForGenerateGlyphMask() &&
-        !aBuilder->IsForPaintingSelectionBG()) {
-      nsDisplayTransform *transformItem =
-        new (aBuilder) nsDisplayTransform(aBuilder, this,
-                                          &resultList, dirtyRect, 0,
-                                          allowAsyncAnimation);
-      resultList.AppendNewToTop(transformItem);
-    }
+    nsDisplayTransform *transformItem =
+      new (aBuilder) nsDisplayTransform(aBuilder, this,
+                                        &resultList, dirtyRect, 0,
+                                        allowAsyncAnimation);
+    resultList.AppendNewToTop(transformItem);
 
     if (hasPerspective) {
       if (clipCapturedBy == ContainerItemType::ePerspective) {

layout/painting/nsDisplayList.cpp

@@ -8087,11 +8087,18 @@ already_AddRefed<Layer> nsDisplayTransform::BuildLayer(nsDisplayListBuilder *aBu
                                                        LayerManager *aManager,
                                                        const ContainerLayerParameters& aContainerParameters)
 {
+  // While generating a glyph mask, the transform vector of the root frame had
+  // been applied into the target context, so stop applying it again here.
+  const bool shouldSkipTransform =
+    (aBuilder->RootReferenceFrame() == mFrame) &&
+    (aBuilder->IsForGenerateGlyphMask() || aBuilder->IsForPaintingSelectionBG());
+
   /* For frames without transform, it would not be removed for
    * backface hidden here.  But, it would be removed by the init
    * function of nsDisplayTransform.
    */
-  const Matrix4x4& newTransformMatrix = GetTransformForRendering();
+  const Matrix4x4 newTransformMatrix =
+    shouldSkipTransform ? Matrix4x4(): GetTransformForRendering();
 
   uint32_t flags = FrameLayerBuilder::CONTAINER_ALLOW_PULL_BACKGROUND_COLOR;
   RefPtr<ContainerLayer> container = aManager->GetLayerBuilder()->