From d83203f9405cfa8517a122e263ddc56ad069cf20 Mon Sep 17 00:00:00 2001
From: Matt Woodrow <mwoodrow@mozilla.com>
Date: Thu, 26 Jul 2012 16:33:31 +1200
Subject: [PATCH] Bug 772296 - Forward transform the overflow rect when
 computing dirty rects for 3d transforms. r=roc

---
 layout/generic/nsFrame.cpp | 37 ++++++++++++++-----------------------
 1 file changed, 14 insertions(+), 23 deletions(-)

diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 37ff5743544..869e7700ce0 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -1778,33 +1778,24 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
         nsDisplayTransform::ShouldPrerenderTransformedContent(aBuilder, this)) {
       dirtyRect = GetVisualOverflowRectRelativeToSelf();
     } else {
-      if (Preserves3D()) {
-        // Preserve3D frames are difficult. Trying to  back-transform arbitrary rects
-        // gives us really weird results. I believe this is from points that lie beyond the
-        // vanishing point. As a workaround we transform the overflow rect into screen space
-        // and compare in that coordinate system.
+      // Trying to  back-transform arbitrary rects gives us really weird results. I believe 
+      // this is from points that lie beyond the vanishing point. As a workaround we transform t
+      // he overflow rect into screen space and compare in that coordinate system.
         
-        // Transform the overflow rect into screen space
-        nsRect overflow = GetVisualOverflowRectRelativeToSelf();
-        nsPoint offset = aBuilder->ToReferenceFrame(this);
-        overflow += offset;
-        overflow = nsDisplayTransform::TransformRect(overflow, this, offset);
+      // Transform the overflow rect into screen space
+      nsRect overflow = GetVisualOverflowRectRelativeToSelf();
+      nsPoint offset = aBuilder->ToReferenceFrame(this);
+      overflow += offset;
+      overflow = nsDisplayTransform::TransformRect(overflow, this, offset);
 
-        dirtyRect += offset;
+      dirtyRect += offset;
 
-        if (dirtyRect.Intersects(overflow)) {
-          dirtyRect = GetVisualOverflowRectRelativeToSelf();
-        } else {
-          dirtyRect.SetEmpty();
-        }
+      if (dirtyRect.Intersects(overflow)) {
+        // If they intersect, we take our whole overflow rect. We could instead take the intersection
+        // and then reverse transform it but I doubt this extra work is worthwhile.
+        dirtyRect = GetVisualOverflowRectRelativeToSelf();
       } else {
-        // Transform dirtyRect into our frame's local coordinate space. Note that
-        // the new value is the bounds of the old value's transformed vertices, so
-        // the area covered by dirtyRect may increase here.
-        if (!nsDisplayTransform::UntransformRect(dirtyRect, this, nsPoint(0, 0), &dirtyRect)) {
-          // we have a singular transform - surely we must be drawing nothing.
-          dirtyRect.SetEmpty();
-        }
+        dirtyRect.SetEmpty();
       }
       if (!Preserves3DChildren() && !dirtyRect.Intersects(GetVisualOverflowRectRelativeToSelf())) {
         return NS_OK;