Enhance tab management by updating the toolbar on tab close and adjusting overflow behavior for the tab browser

src/browser/base/content/ZenUIManager.mjs

@@ -22,6 +22,8 @@ var gZenUIManager = {
         this.sidebarHeightThrottle
       )
     ).observe(document.getElementById('navigator-toolbox'));
+
+    window.addEventListener('TabClose', this.updateTabsToolbar.bind(this));
   },
 
   updateTabsToolbar() {

src/browser/base/zen-components/ZenPinnedTabManager.mjs

@@ -572,6 +572,7 @@
         gBrowser.pinTab(tab);
         this.onTabIconChanged(tab);
       }
+      gZenUIManager.updateTabsToolbar();
     }
 
     removeEssentials() {
@@ -584,6 +585,7 @@
         }
         gBrowser.unpinTab(tab);
       }
+      gZenUIManager.updateTabsToolbar();
     }
 
     _insertItemsIntoTabContextMenu() {

src/toolkit/content/widgets/arrowscrollbox-js.patch

@@ -1,19 +1,16 @@
 diff --git a/toolkit/content/widgets/arrowscrollbox.js b/toolkit/content/widgets/arrowscrollbox.js
-index 328c770d275ebbaada8a44438eaf738b1a62d985..c7d178295745ed975f5a3e5dea028dbeeece1704 100644
+index 328c770d275ebbaada8a44438eaf738b1a62d985..95460108c6356408170b8a4a40d55a8f0621756b 100644
 --- a/toolkit/content/widgets/arrowscrollbox.js
 +++ b/toolkit/content/widgets/arrowscrollbox.js
-@@ -121,8 +121,10 @@
-           );
-         });
-       });
-+      if (this.id !== 'tabbrowser-arrowscrollbox') {
-       overflowObserver.observe(slot);
-       overflowObserver.observe(this.scrollbox);
-+      }
-     }
+@@ -98,6 +98,7 @@
  
-     connectedCallback() {
-@@ -639,7 +641,7 @@
+       let slot = this.shadowRoot.querySelector("slot");
+       let overflowObserver = new ResizeObserver(_ => {
++        if (this.id == 'tabbrowser-arrowscrollbox') return; // zen: do NOT underflow/overflow on tabbrowser-arrowscrollbox
+         let contentSize =
+           slot.getBoundingClientRect()[this.#verticalMode ? "height" : "width"];
+         // NOTE(emilio): This should be contentSize > scrollClientSize, but due
+@@ -639,7 +640,7 @@
  
      on_wheel(event) {
        // Don't consume the event if we can't scroll.