From b92d6976575790ed4a7535054b166e04faf2c617 Mon Sep 17 00:00:00 2001
From: Andrey Bochkarev <50177704+octaviusz@users.noreply.github.com>
Date: Wed, 25 Feb 2026 22:37:53 +0300
Subject: [PATCH] fix: Add guard to prevent invalid split drops, p=#12511

---
 src/zen/drag-and-drop/ZenDragAndDrop.js | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/zen/drag-and-drop/ZenDragAndDrop.js b/src/zen/drag-and-drop/ZenDragAndDrop.js
index c30d5548d..af2ed7a61 100644
--- a/src/zen/drag-and-drop/ZenDragAndDrop.js
+++ b/src/zen/drag-and-drop/ZenDragAndDrop.js
@@ -683,6 +683,10 @@
 
       const dt = event.dataTransfer;
       const draggedTab = dt.mozGetDataAt(TAB_DROP_TYPE, 0);
+      if (!isTab(draggedTab)) {
+        return;
+      }
+
       const dragData = draggedTab._dragData;
       const movingTabsSet = dragData.movingTabsSet;
       const dropElement = event.target.closest(".tabbrowser-tab");
@@ -771,6 +775,7 @@
       }
 
       this.#dragOverSplit.fakeTab = element;
+      this.#dragOverSplit.canDrop = true;
     }
 
     _clearDragOverSplit() {
@@ -782,6 +787,7 @@
       this.#dragOverSplit.timer = null;
       this.#dragOverSplit.fakeTab = null;
       this.#dragOverSplit.data = null;
+      this.#dragOverSplit.canDrop = null;
     }
 
     handle_windowDragEnter(event) {
@@ -881,6 +887,10 @@
     }
 
     #handle_dropCreateSplit(event) {
+      if (!this.#dragOverSplit.canDrop) {
+        return;
+      }
+
       const dragData = this.#dragOverSplit.data;
       const dt = event.dataTransfer;
       const draggedTab = dt.mozGetDataAt(TAB_DROP_TYPE, 0);
@@ -1118,7 +1128,7 @@
     // eslint-disable-next-line complexity
     #applyDragoverIndicator(event, dropElement, movingTabs, draggedTab) {
       // Doesn't show indicator when dragOverSplit
-      if (this.#dragOverSplit.data) {
+      if (this.#dragOverSplit.canDrop) {
         return;
       }
       const separation = 4;