Added support to unsplit tabs when dragging them to the sidebar

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

@@ -1588,7 +1588,9 @@ class ZenViewSplitter extends ZenDOMOperatedFeature {
         this.splitTabs([draggedTab, droppedOnTab], gridType, 1);
       }
     }
+    window.requestAnimationFrame(() => {
       this._maybeRemoveFakeBrowser(false);
+    });
 
     if (browserContainer) {
       gZenUIManager.motion
@@ -1606,12 +1608,39 @@ class ZenViewSplitter extends ZenDOMOperatedFeature {
           }
         )
         .then(() => {
-          gBrowser.tabbox.removeAttribute('style');
+          this._maybeRemoveFakeBrowser(false);
         });
     }
     return true;
   }
 
+  handleTabDrop(event, urls, replace, inBackground) {
+    if (!inBackground || replace || urls.length !== 1) {
+      return false;
+    }
+    const url = urls[0];
+    if (!url.startsWith('panel-')) {
+      return false;
+    }
+    const browserContainer = document.getElementById(url);
+    const browser = browserContainer?.querySelector('browser');
+    if (!browser) {
+      return false;
+    }
+    const tab = gBrowser.getTabForBrowser(browser);
+    if (!tab) {
+      return false;
+    }
+    if (tab.splitView) {
+      // Unsplit the tab and exit from the drag view
+      this.dropZone?.removeAttribute('enabled');
+      this.disableTabRearrangeView(event);
+      this.removeTabFromSplit(browserContainer);
+      return true;
+    }
+    return false;
+  }
+
   /**
    * Gets or creates a tab group for split view tabs
    * @param {Array} tabs Initial tabs to add to the group if creating new

src/browser/components/tabbrowser/content/tabs-js.patch

@@ -1,5 +1,5 @@
 diff --git a/browser/components/tabbrowser/content/tabs.js b/browser/components/tabbrowser/content/tabs.js
-index fa96568d366fd3608f9bd583fa793150bd815c8b..9151fa411faa9689c7b079fafe0fab1234177f40 100644
+index fa96568d366fd3608f9bd583fa793150bd815c8b..a2a0ca3982634252abfdad20e768431a840ea19b 100644
 --- a/browser/components/tabbrowser/content/tabs.js
 +++ b/browser/components/tabbrowser/content/tabs.js
 @@ -94,7 +94,7 @@
@@ -97,6 +97,15 @@ index fa96568d366fd3608f9bd583fa793150bd815c8b..9151fa411faa9689c7b079fafe0fab12
                  postTransitionCleanup();
                } else {
                  let onTransitionEnd = transitionendEvent => {
+@@ -1249,7 +1263,7 @@
+               return;
+             }
+           }
+-
++          if (gZenViewSplitter.handleTabDrop(event, urls, replace, inBackground)) return;
+           gBrowser.loadTabs(urls, {
+             inBackground,
+             replace,
 @@ -1279,13 +1293,23 @@
          return;
        }