feat: Always make sure to save the last closed window to the sidebar object, b=no-bug, c=no-component

src/browser/components/sessionstore/SessionStore-sys-mjs.patch

@@ -1,5 +1,5 @@
 diff --git a/browser/components/sessionstore/SessionStore.sys.mjs b/browser/components/sessionstore/SessionStore.sys.mjs
-index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520bd2b354b8 100644
+index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..cc9185308c83abfa173041a10edbcb9bbfa3d264 100644
 --- a/browser/components/sessionstore/SessionStore.sys.mjs
 +++ b/browser/components/sessionstore/SessionStore.sys.mjs
 @@ -127,6 +127,8 @@ const TAB_EVENTS = [
@@ -81,7 +81,15 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
            this.maybeSaveClosedWindow(winData, isLastWindow);
  
            if (!isLastWindow && winData.closedId > -1) {
-@@ -3373,7 +3390,7 @@ var SessionStoreInternal = {
+@@ -2582,6 +2599,7 @@ var SessionStoreInternal = {
+       let alreadyStored = winIndex != -1;
+       // If sidebar command is truthy, i.e. sidebar is open, store sidebar settings
+       let shouldStore = hasSaveableTabs || isLastWindow;
++      lazy.ZenSessionStore.maybeSaveClosedWindow(winData, isLastWindow);
+ 
+       if (shouldStore && !alreadyStored) {
+         let index = this._closedWindows.findIndex(win => {
+@@ -3373,7 +3391,7 @@ var SessionStoreInternal = {
      if (!isPrivateWindow && tabState.isPrivate) {
        return;
      }
@@ -90,7 +98,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
        return;
      }
  
-@@ -4089,6 +4106,12 @@ var SessionStoreInternal = {
+@@ -4089,6 +4107,12 @@ var SessionStoreInternal = {
          Math.min(tabState.index, tabState.entries.length)
        );
        tabState.pinned = false;
@@ -103,7 +111,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
  
        if (inBackground === false) {
          aWindow.gBrowser.selectedTab = newTab;
-@@ -4525,6 +4548,7 @@ var SessionStoreInternal = {
+@@ -4525,6 +4549,7 @@ var SessionStoreInternal = {
        // Append the tab if we're opening into a different window,
        tabIndex: aSource == aTargetWindow ? pos : Infinity,
        pinned: state.pinned,
@@ -111,7 +119,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
        userContextId: state.userContextId,
        skipLoad: true,
        preferredRemoteType,
-@@ -5374,7 +5398,7 @@ var SessionStoreInternal = {
+@@ -5374,7 +5399,7 @@ var SessionStoreInternal = {
  
      for (let i = tabbrowser.pinnedTabCount; i < tabbrowser.tabs.length; i++) {
        let tab = tabbrowser.tabs[i];
@@ -120,7 +128,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
          removableTabs.push(tab);
        }
      }
-@@ -5483,7 +5507,7 @@ var SessionStoreInternal = {
+@@ -5483,7 +5508,7 @@ var SessionStoreInternal = {
  
      // collect the data for all windows
      for (ix in this._windows) {
@@ -129,7 +137,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
          // window data is still in _statesToRestore
          continue;
        }
-@@ -5625,11 +5649,15 @@ var SessionStoreInternal = {
+@@ -5625,11 +5650,15 @@ var SessionStoreInternal = {
      }
  
      let tabbrowser = aWindow.gBrowser;
@@ -146,7 +154,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
      // update the internal state data for this window
      for (let tab of tabs) {
        if (tab == aWindow.FirefoxViewHandler.tab) {
-@@ -5652,7 +5680,7 @@ var SessionStoreInternal = {
+@@ -5652,7 +5681,7 @@ var SessionStoreInternal = {
      // a window is closed, point to the first item in the tab strip instead (it will never be the Firefox View tab,
      // since it's only inserted into the tab strip after it's selected).
      if (aWindow.FirefoxViewHandler.tab?.selected) {
@@ -155,7 +163,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
        winData.title = tabbrowser.tabs[0].label;
      }
      winData.selected = selectedIndex;
-@@ -5765,8 +5793,8 @@ var SessionStoreInternal = {
+@@ -5765,8 +5794,8 @@ var SessionStoreInternal = {
      // selectTab represents.
      let selectTab = 0;
      if (overwriteTabs) {
@@ -166,7 +174,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
        selectTab = Math.min(selectTab, winData.tabs.length);
      }
  
-@@ -5809,6 +5837,9 @@ var SessionStoreInternal = {
+@@ -5809,6 +5838,9 @@ var SessionStoreInternal = {
          winData.tabs,
          winData.groups ?? []
        );
@@ -176,7 +184,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
        this._log.debug(
          `restoreWindow, createTabsForSessionRestore returned ${tabs.length} tabs`
        );
-@@ -6372,6 +6403,25 @@ var SessionStoreInternal = {
+@@ -6372,6 +6404,25 @@ var SessionStoreInternal = {
  
      // Most of tabData has been restored, now continue with restoring
      // attributes that may trigger external events.
@@ -202,7 +210,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
  
      if (tabData.pinned) {
        tabbrowser.pinTab(tab);
-@@ -7290,7 +7340,7 @@ var SessionStoreInternal = {
+@@ -7290,7 +7341,7 @@ var SessionStoreInternal = {
  
        let groupsToSave = new Map();
        for (let tIndex = 0; tIndex < window.tabs.length; ) {
@@ -211,7 +219,7 @@ index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..89ae48f816bee85b2870f383723b520b
            // Adjust window.selected
            if (tIndex + 1 < window.selected) {
              window.selected -= 1;
-@@ -7305,7 +7355,7 @@ var SessionStoreInternal = {
+@@ -7305,7 +7356,7 @@ var SessionStoreInternal = {
            );
            // We don't want to increment tIndex here.
            continue;

src/zen/sessionstore/ZenSessionManager.sys.mjs

@@ -156,6 +156,24 @@ export class nsZenSessionManager {
     this.log(`Saving Zen session data with ${this.#sidebar.tabs?.length || 0} tabs`);
   }
 
+  /**
+   * Saves the session data for a closed window if it meets the criteria.
+   * See SessionStoreInternal.maybeSaveClosedWindow for more details.
+   *
+   * @param aWinData - The window data object to save.
+   * @param isLastWindow - Whether this is the last saveable window.
+   */
+  maybeSaveClosedWindow(aWinData, isLastWindow) {
+    // We only want to save the *last* normal window that is closed.
+    // If its not the last window, we can still update the sidebar object
+    // based on other open windows.
+    if (aWinData.isPopup || aWinData.isTaskbarTab || aWinData.isZenUnsynced || !isLastWindow) {
+      return;
+    }
+    this.log('Saving closed window session data into Zen session store');
+    this.saveState({ windows: [aWinData] });
+  }
+
   /**
    * Collects session data for a given window.
    *