Revert "Bug 1965343 - Adopt whole tab group at once in replaceGroupWithWindow r=tabbrowser-reviewers,sthompson" for causing bc failures @browser_tab_groups.js.

This reverts commit fac3f86e95.

browser/components/tabbrowser/content/tabbrowser.js

@@ -5729,15 +5729,11 @@
     /**
      * Moves a tab to a new browser window, unless it's already the only tab
      * in the current window, in which case this will do nothing.
-     *
-     * @param {MozTabbrowserTab|MozTabbrowserTabGroup|MozTabbrowserTabGroup.labelElement} aTab
      */
     replaceTabWithWindow(aTab, aOptions) {
       if (this.tabs.length == 1) {
         return null;
       }
-      // If aTab is a tab group, callers should check that it is not the only
-      // element in its window.
 
       var options = "chrome,dialog=no,all";
       for (var name in aOptions) {
@@ -5750,7 +5746,7 @@
 
       // Play the tab closing animation to give immediate feedback while
       // waiting for the new window to appear.
-      if (!gReduceMotion && this.isTab(aTab)) {
+      if (!gReduceMotion) {
         aTab.style.maxWidth = ""; // ensure that fade-out transition happens
         aTab.removeAttribute("fadein");
       }
@@ -5845,7 +5841,42 @@
      *   The tab group to move.
      */
     replaceGroupWithWindow(group) {
-      return this.replaceTabWithWindow(group);
+      // The first tab added to the new window will be selected.
+      // If a tab in the group is selected, adopt it first.
+      let selectedIndex = group.tabs.indexOf(gBrowser.selectedTab);
+      if (selectedIndex < 0) {
+        // Otherwise, we'll first adopt the first tab in the group
+        selectedIndex = 0;
+      }
+      let firstTab = group.tabs[selectedIndex];
+      group.removedByAdoption = true;
+      let newWindow = this.replaceTabWithWindow(firstTab);
+
+      newWindow.addEventListener(
+        "before-initial-tab-adopted",
+        () => {
+          let tabsToGroup = group.tabs.map((tab, i) => {
+            // addtabGroup will handle adopting the other tabs, but we already
+            // started adopting the tab at selectedIndex so we need to swap
+            // the old tab out for the new one.
+            if (i == selectedIndex) {
+              return newWindow.gBrowser.visibleTabs[0];
+            }
+            return tab;
+          });
+          // The initial tab isn't fully adopted yet, but the tab object has been
+          // instantiated, so we can make a group now.
+          newWindow.gBrowser.addTabGroup(tabsToGroup, {
+            id: group.id,
+            label: group.label,
+            color: group.color,
+            isAdoptingGroup: true,
+          });
+          Glean.tabgroup.groupInteractions.move_window.add(1);
+        },
+        { once: true }
+      );
+      return newWindow;
     }
 
     /**