Bug 1412456 - Change browser element's primary attribute immediately when tab change requested (r=mconley)

Currently the e10s tab switcher doesn't set the primary="true"
attribute on a browser element until the tab switcher is
destroyed. This means that using the |content| global is unreliable in
e10s since it may take as long as the tab switcher's unload delay
before the attribute changes. Normally this doesn't matter since
|content| isn't useful in e10s. However, tests that use tabs like
about:preferences rely on |content|, and there isn't a good reason to
fix them. So let's just change the attribute immediately upon changing
tabs.

MozReview-Commit-ID: 1cgJbmXD6of

browser/base/content/tabbrowser.xml

@@ -4301,10 +4301,10 @@
 
             // Auxilliary state variables:
 
-            visibleTab: this.selectedTab,   // Tab that's on screen.
-            spinnerTab: null,               // Tab showing a spinner.
-            blankTab: null,                 // Tab showing blank.
-            originalTab: this.selectedTab,  // Tab that we started on.
+            visibleTab: this.selectedTab,      // Tab that's on screen.
+            spinnerTab: null,                  // Tab showing a spinner.
+            blankTab: null,                    // Tab showing blank.
+            lastPrimaryTab: this.selectedTab,  // Tab with primary="true"
 
             tabbrowser: this,  // Reference to gBrowser.
             loadTimer: null,   // TAB_SWITCH_TIMEOUT nsITimer instance.
@@ -4485,17 +4485,6 @@
 
               this.destroy();
 
-              let toBrowser = this.requestedTab.linkedBrowser;
-              toBrowser.setAttribute("primary", "true");
-
-              let fromBrowser = this.originalTab.linkedBrowser;
-              // It's possible that the tab we're switching from closed
-              // before we were able to finalize, in which case, fromBrowser
-              // doesn't exist.
-              if (fromBrowser) {
-                fromBrowser.removeAttribute("primary");
-              }
-
               document.commandDispatcher.unlock();
 
               let event = new CustomEvent("TabSwitchDone", {
@@ -4673,6 +4662,9 @@
               if (this.lastVisibleTab && !this.lastVisibleTab.linkedBrowser) {
                 this.lastVisibleTab = null;
               }
+              if (this.lastPrimaryTab && !this.lastPrimaryTab.linkedBrowser) {
+                this.lastPrimaryTab = null;
+              }
               if (this.blankTab && !this.blankTab.linkedBrowser) {
                 this.blankTab = null;
               }
@@ -5051,6 +5043,12 @@
 
               this.requestedTab = tab;
 
+              tab.linkedBrowser.setAttribute("primary", "true");
+              if (this.lastPrimaryTab && this.lastPrimaryTab != tab) {
+                this.lastPrimaryTab.linkedBrowser.removeAttribute("primary");
+              }
+              this.lastPrimaryTab = tab;
+
               this.suppressDisplayPortAndQueueUnload(this.requestedTab, this.UNLOAD_DELAY);
               this._requestingTab = false;
             },