DirectShow Video Mixing Renderer (VMR-7) filter exhibits a (regression?) bug in Windows 10 systems. When aspect ratio preservation is enabled in VMR_ARMODE_LETTER_BOX mode, which makes overall sense as default mode quote so often, the letterboxing does not work as expected.
The problem is easy to reproduce with a well known DShowPlayerSDK sample application, with an edit enforcing VMR-7 mode. Once video is started, just resize the window and the parts not covered by video will not be erased as expected.
Apparently this worked well earlier.