From 3ffcee093773fb48820f0ba57ef42175b6638716 Mon Sep 17 00:00:00 2001
From: "L. David Baron" <dbaron@dbaron.org>
Date: Fri, 14 Jan 2011 19:57:53 -0800
Subject: [PATCH] Don't cancel transitions that are almost completed (and round
 to their final value) when we get an unrelated style change.  (Bug 613888) 
 r=bzbarsky  a2.0=blocking

---
 layout/style/nsTransitionManager.cpp | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/layout/style/nsTransitionManager.cpp b/layout/style/nsTransitionManager.cpp
index de0a9412b7c1..166e467ebd23 100644
--- a/layout/style/nsTransitionManager.cpp
+++ b/layout/style/nsTransitionManager.cpp
@@ -588,9 +588,11 @@ nsTransitionManager::ConsiderStartingTransition(nsCSSProperty aProperty,
 
   ElementPropertyTransition pt;
   nsStyleAnimation::Value dummyValue;
-  PRBool shouldAnimate =
+  PRBool haveValues =
     TransExtractComputedValue(aProperty, aOldStyleContext, pt.mStartValue) &&
-    TransExtractComputedValue(aProperty, aNewStyleContext, pt.mEndValue) &&
+    TransExtractComputedValue(aProperty, aNewStyleContext, pt.mEndValue);
+  PRBool shouldAnimate =
+    haveValues &&
     pt.mStartValue != pt.mEndValue &&
     // Check that we can interpolate between these values
     // (If this is ever a performance problem, we could add a
@@ -613,13 +615,19 @@ nsTransitionManager::ConsiderStartingTransition(nsCSSProperty aProperty,
   nsPresContext *presContext = aNewStyleContext->PresContext();
 
   if (!shouldAnimate) {
-    if (currentIndex != nsTArray<ElementPropertyTransition>::NoIndex) {
+    nsTArray<ElementPropertyTransition> &pts =
+      aElementTransitions->mPropertyTransitions;
+    if (currentIndex != nsTArray<ElementPropertyTransition>::NoIndex &&
+        (!haveValues || pts[currentIndex].mEndValue != pt.mEndValue)) {
       // We're in the middle of a transition, but just got a
       // non-transition style change changing to exactly the
       // current in-progress value.   (This is quite easy to cause
       // using 'transition-delay'.)
-      nsTArray<ElementPropertyTransition> &pts =
-        aElementTransitions->mPropertyTransitions;
+      //
+      // We also check that this current in-progress value is different
+      // from the end value; we don't want to cancel a transition that
+      // is almost done (and whose current value rounds to its end
+      // value) just because we got an unrelated style change.
       pts.RemoveElementAt(currentIndex);
       if (pts.IsEmpty()) {
         aElementTransitions->Destroy();