Bug 1478112 - Get rid of preselect tabbox workaround, and adjust focus after changing visible tab right away. r=dao

The "select" event handler workaround was originally added in bug 1379270 to make it
possible to focus and select the URL bar a little bit later. This ugly hack was to
workaround an issue with WebExtensions that override about:newtab with the
chrome_url_overrides property (the issue would be that the URL bar would not be
properly focused and selected if about:newtab was overridden).

Back in the day, this was necessary because the overriding URL was fully displayed
in the URL bar (moz-webextension://...). These days, when about:newtab is overridden,
the URL bar is still empty - we just end up showing the information about the
WebExtension overriding about:newtab to the left of the URL bar.

So I think we can remove the old workaround.

Differential Revision: https://phabricator.services.mozilla.com/D3447

--HG--
extra : moz-landing-system : lando

browser/base/content/tabbrowser.js

@@ -874,10 +874,16 @@ window._gBrowser = {
       return;
     }
 
-    if (!aForceUpdate) {
-      document.commandDispatcher.lock();
+    let newTab = this.getTabForBrowser(newBrowser);
 
+    if (!aForceUpdate) {
       TelemetryStopwatch.start("FX_TAB_SWITCH_UPDATE_MS");
+
+      if (gMultiProcessBrowser) {
+        this._getSwitcher().requestTab(newTab);
+      }
+
+      document.commandDispatcher.lock();
     }
 
     let oldTab = this.selectedTab;
@@ -904,7 +910,6 @@ window._gBrowser = {
           !window.isFullyOccluded);
     }
 
-    let newTab = this.getTabForBrowser(newBrowser);
     this._selectedBrowser = newBrowser;
     this._selectedTab = newTab;
     this.showTab(newTab);
@@ -4352,12 +4357,6 @@ window._gBrowser = {
       }
     });
 
-    this.tabpanels.addEventListener("preselect", event => {
-      if (gMultiProcessBrowser) {
-        this._getSwitcher().requestTab(event.detail);
-      }
-    });
-
     this.addEventListener("DOMWindowClose", (event) => {
       if (!event.isTrusted)
         return;

browser/modules/AsyncTabSwitcher.jsm

@@ -438,21 +438,7 @@ class AsyncTabSwitcher {
             TelemetryStopwatch.cancel("FX_TAB_SWITCH_COMPOSITE_E10S_MS", this.window);
           }
 
-          if (this._requestingTab) {
-            /*
-             * If _requestingTab is set, that means that we're switching the
-             * visibility of the tab synchronously, and we need to wait for
-             * the "select" event before shifting focus so that
-             * _adjustFocusAfterTabSwitch runs with the right information for
-             * the tab switch.
-             */
-            this.tabbrowser.addEventListener("select", () => {
-              this.tabbrowser._adjustFocusAfterTabSwitch(showTab);
-            }, { once: true });
-          } else {
-            this.tabbrowser._adjustFocusAfterTabSwitch(showTab);
-          }
-
+          this.tabbrowser._adjustFocusAfterTabSwitch(showTab);
           this.maybeActivateDocShell(this.requestedTab);
         }
       }
@@ -1006,7 +992,6 @@ class AsyncTabSwitcher {
         .add(warmingState);
     }
 
-    this._requestingTab = true;
     this.logState("requestTab " + this.tinfo(tab));
     this.startTabSwitch();
 
@@ -1025,7 +1010,6 @@ class AsyncTabSwitcher {
     this.lastPrimaryTab = tab;
 
     this.queueUnload(this.UNLOAD_DELAY);
-    this._requestingTab = false;
   }
 
   queueUnload(unloadTimeout) {

toolkit/content/widgets/tabbox.xml

@@ -614,11 +614,6 @@
           if (val < 0 || val >= this.children.length)
             return val;
 
-          // Give the tabbrowser a chance to run logic regardless of
-          // whether the panel is going to change:
-          this.dispatchEvent(new CustomEvent("preselect",
-            { detail: this.getRelatedElement(this.children[val]) }));
-
           var panel = this._selectedPanel;
           this._selectedPanel = this.children[val];
           this.setAttribute("selectedIndex", val);