Bug 1198135 - Part 3: Set the size of HTMLScrollFrames earlier so that we compute perspective earlier. r=dbaron

layout/generic/nsGfxScrollFrame.cpp

@@ -1034,6 +1034,16 @@ nsHTMLScrollFrame::Reflow(nsPresContext*           aPresContext,
 
   ReflowContents(&state, aDesiredSize);
 
+  aDesiredSize.Width() = state.mInsideBorderSize.width +
+    state.mComputedBorder.LeftRight();
+  aDesiredSize.Height() = state.mInsideBorderSize.height +
+    state.mComputedBorder.TopBottom();
+
+  // Set the size of the frame now since computing the perspective-correct
+  // overflow (within PlaceScrollArea) can rely on it.
+  SetSize(aDesiredSize.GetWritingMode(),
+          aDesiredSize.Size(aDesiredSize.GetWritingMode()));
+
   // Restore the old scroll position, for now, even if that's not valid anymore
   // because we changed size. We'll fix it up in a post-reflow callback, because
   // our current size may only be temporary (e.g. we're compute XUL desired sizes).
@@ -1073,11 +1083,6 @@ nsHTMLScrollFrame::Reflow(nsPresContext*           aPresContext,
     }
   }
 
-  aDesiredSize.Width() = state.mInsideBorderSize.width +
-    state.mComputedBorder.LeftRight();
-  aDesiredSize.Height() = state.mInsideBorderSize.height +
-    state.mComputedBorder.TopBottom();
-
   aDesiredSize.SetOverflowAreasToDesiredBounds();
   if (mHelper.IsIgnoringViewportClipping()) {
     aDesiredSize.mOverflowAreas.UnionWith(