Bug 940262 - [Australis] Hide .tab-background in the early tabopen animation phase and show it after a short offset r=dao

From b7ac00600af262042d076ba5ac04138eb4ee0418 Mon Sep 17 00:00:00 2001

browser/base/content/browser.css

@@ -119,6 +119,19 @@ tabbrowser {
               max-width 230ms ease-out;
 }
 
+.tab-background {
+  /* Explicitly set the visibility to override the value (collapsed)
+   * we inherit from #TabsToolbar[collapsed] upon opening a browser window. */
+  visibility: visible;
+  /* This transition is only applied when opening a new tab. Closing tabs
+   * are just hidden so we don't need to adjust the delay for that. */
+  transition: visibility 0ms 25ms;
+}
+
+.tab-background[selected]:not([fadein]):not([pinned]) {
+  visibility: hidden;
+}
+
 .tab-throbber:not([fadein]):not([pinned]),
 .tab-label:not([fadein]):not([pinned]),
 .tab-icon-image:not([fadein]):not([pinned]),

browser/base/content/tabbrowser.xml

@@ -4643,7 +4643,7 @@
 
     <content context="tabContextMenu" closetabtext="&closeTab.label;">
       <xul:stack class="tab-stack" flex="1">
-        <xul:hbox xbl:inherits="pinned,selected,titlechanged"
+        <xul:hbox xbl:inherits="pinned,selected,titlechanged,fadein"
                   class="tab-background">
           <xul:hbox xbl:inherits="pinned,selected,titlechanged"
                     class="tab-background-start"/>