fix: prevent misplaced scrollable shadow when video gallery is loading

[ArturGaspar]

Description

Prevent ugly shadow when video gallery is loading.

Before:

After:

Testing instructions

1. (Optional) Create a mock videos response with a useful delay
   1. Enable mock video uploads (create waffle flag contentstore.mock_video_uploads enabled for everyone)
   2. Modify videos_index_json() to have a delay so the loading phase can be observed more carefully, as follows:

      def videos_index_json(course):
          """
          ...
          """
          import time
          time.sleep(20)
          # index_videos, __ = _get_index_videos(course)
          index_videos = [{
              'edx_video_id': 'aaaaaaaa-aaaa-4aaa-aaaa-aaaaaaaaaaaa',
              'client_video_id': 'video.mp4',
              'created': '1970-01-01T00:00:00Z',
              'duration': 42.5,
              'status': 'upload',
              'course_video_image_url': 'https://video/images/1234.jpg'
          }]
          return JsonResponse({"videos": index_videos}, status=200)
      

2. Open video gallery
   1. Enable the new video editor (create waffle flags new_core_editors.use_new_video_editor and new_core_editors.use_video_gallery_flow enabled for everyone)
   2. Create a course unit with a video
   3. Edit the video (should open in new editor)
   4. Click "< Replace video" to go to the gallery
3. See that the shadow does not appear while the gallery is loading

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