WebUI Tab Strip: Don't update drag session on dragenter events

Previously, the drag session would be updated as soon as a dragenter
event was fired. This was to allow a dragged out tab to instantly
re-appear in the tab strip once it was dragged back in. Unfortunately,
there exists a bug that causes the drag event's target to be incorrect
if the scale is not 100%.

Removing the update on dragenter should not cause any issues because
the drag session will get updated on the very next dragover event.

Bug: 1117190
Change-Id: Ia06007ab4c3c4827b2783be9cef4df1c30d00f8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357599Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799219}

chrome/browser/resources/tab_strip/drag_manager.js

@@ -512,7 +512,9 @@ export class DragManager {
   /** @param {!DragEvent} event */
   onDragEnter_(event) {
     if (this.dragSession_) {
-      this.dragSession_.update(event);
+      // TODO(crbug.com/843556): Do not update the drag session on dragenter.
+      // An incorrect event target on dragenter causes tabs to move around
+      // erroneously.
       return;
     }
 

chrome/test/data/webui/tab_strip/drag_manager_test.js

@@ -638,7 +638,7 @@ suite('DragManager', () => {
     delegate.dispatchEvent(new DragEvent('dragleave', {dataTransfer}));
     assertTrue(isDraggedOut);
 
-    delegate.dispatchEvent(new DragEvent('dragenter', {dataTransfer}));
+    delegate.dispatchEvent(new DragEvent('dragover', {dataTransfer}));
     assertFalse(isDraggedOut);
   });
 });