Handle the case of two scroll events cancelling each other out.

R=pkasting
BUG=none
TEST=covered by unittests

Review URL: http://codereview.chromium.org/491022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@34314 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 21 insertions, 0 deletions

diff --git a/chrome/renderer/paint_aggregator.cc b/chrome/renderer/paint_aggregator.cc
index 125f7b5..c5c4405 100644
--- a/chrome/renderer/paint_aggregator.cc
+++ b/chrome/renderer/paint_aggregator.cc
@@ -155,6 +155,12 @@ void PaintAggregator::ScrollRect(int dx, int dy, const gfx::Rect& clip_rect) {
   update_.scroll_rect = clip_rect;
   update_.scroll_delta.Offset(dx, dy);
 
+  // We might have just wiped out a pre-existing scroll.
+  if (update_.scroll_delta == gfx::Point()) {
+    update_.scroll_rect = gfx::Rect();
+    return;
+  }
+
   // Adjust any contained paint rects and check for any overlapping paints.
   for (size_t i = 0; i < update_.paint_rects.size(); ++i) {
     if (update_.scroll_rect.Contains(update_.paint_rects[i])) {
diff --git a/chrome/renderer/paint_aggregator_unittest.cc b/chrome/renderer/paint_aggregator_unittest.cc
index be7f72a..25503c8 100644
--- a/chrome/renderer/paint_aggregator_unittest.cc
+++ b/chrome/renderer/paint_aggregator_unittest.cc
@@ -87,6 +87,21 @@ TEST(PaintAggregator, DoubleOverlappingScroll) {
   EXPECT_EQ(expected_damage, resulting_damage);
 }
 
+TEST(PaintAggregator, NegatingScroll) {
+  PaintAggregator greg;
+
+  // Scroll twice in opposite directions by equal amounts.  The result
+  // should be no scrolling.
+
+  gfx::Rect rect(1, 2, 3, 4);
+  gfx::Point delta1(1, 0);
+  gfx::Point delta2(-1, 0);
+  greg.ScrollRect(delta1.x(), delta1.y(), rect);
+  greg.ScrollRect(delta2.x(), delta2.y(), rect);
+
+  EXPECT_FALSE(greg.HasPendingUpdate());
+}
+
 TEST(PaintAggregator, DiagonalScroll) {
   PaintAggregator greg;