Bug 776928 - When doing lazy tab restoration, set the <browser>s in question to display:none until restored. r=dao

browser/base/content/tabbrowser.css

@@ -53,3 +53,13 @@ tabpanels {
 .tabbrowser-tabs:not(:hover) > .tabbrowser-arrowscrollbox > .closing-tabs-spacer {
   transition: width .15s ease-out;
 }
+
+/**
+ * Optimization for tabs that are restored lazily. We can save a good amount of
+ * memory that to-be-restored tabs would otherwise consume simply by setting
+ * their browsers to 'display: none' as that will prevent them from having to
+ * create a presentation and the like.
+ */
+browser[pending] {
+  display: none;
+}

browser/components/sessionstore/src/SessionStore.jsm

@@ -2987,6 +2987,7 @@ let SessionStoreInternal = {
       // a tab gets closed before it's been properly restored
       browser.__SS_data = tabData;
       browser.__SS_restoreState = TAB_STATE_NEEDS_RESTORE;
+      browser.setAttribute("pending", "true");
       tab.setAttribute("pending", "true");
 
       // Make sure that set/getTabValue will set/read the correct data by
@@ -3171,6 +3172,7 @@ let SessionStoreInternal = {
 
     // Set this tab's state to restoring
     browser.__SS_restoreState = TAB_STATE_RESTORING;
+    browser.removeAttribute("pending");
     aTab.removeAttribute("pending");
 
     // Remove the history listener, since we no longer need it once we start restoring
@@ -4335,6 +4337,9 @@ let SessionStoreInternal = {
     // The browser is no longer in any sort of restoring state.
     delete browser.__SS_restoreState;
 
+    aTab.removeAttribute("pending");
+    browser.removeAttribute("pending");
+
     // We want to decrement window.__SS_tabsToRestore here so that we always
     // decrement it AFTER a tab is done restoring or when a tab gets "reset".
     window.__SS_tabsToRestore--;