Bug 1400259 - Add will-change:transform to the panel in order to work around a compositor bug. r=mconley

The bug has something to do with ContainerLayer nesting changes being mishandled:
a new ContainerLayer for the transform is being inserted around the container
layer for the opacity, which has an intermediate surface.
This change makes the outer ContainerLayer permanent so that the dynamic
insertion case is not hit.

MozReview-Commit-ID: lETpsr4YJi

--HG--
extra : rebase_source : 7b82976c7c91328c72b54a931732447d82a3ce6d

browser/base/content/browser.css

@@ -1172,6 +1172,7 @@ toolbarpaletteitem[place="palette"] > #downloads-button[indicator] > #downloads-
 
 #BMB_bookmarksPopup:not([animate="false"]) {
   opacity: 0;
+  will-change: transform; /* workaround for bug 1414033 */
   transform: translateY(-70px);
   transition-property: transform, opacity;
   transition-duration: 0.18s, 0.18s;

toolkit/content/xul.css

@@ -481,6 +481,7 @@ panel[type="arrow"][side][animate="cancel"] {
 
 panel[type="arrow"][side]:not([animate="false"]) {
   opacity: 0;
+  will-change: transform; /* workaround for bug 1414033 */
   transform: translateY(-70px);
   transition-property: transform, opacity;
   transition-duration: 0.18s, 0.18s;