MKV playback shows green flicker in Parole on Debian 13 (X11, AMD RX 6800)

When playing MKV files in Parole, video output flickers green. MP4 files play fine. The issue persists regardless of video output setting (X11/Xv, X11/No Xv, Automatic) and even when forcing OpenGL (GST_VIDEO_SINK=glimagesink). VAAPI plugins are not loaded, so hardware acceleration is not in use. The flicker appears to be specific to MKV container handling in GStreamer 1.26.x.

Steps to Reproduce:

1. Install Parole on Debian 13 (trixie).
2. Open Parole → Preferences → Display → set Video Output to either “X Window System (X11/XShm/Xv)” or “X Window System (No Xv)”.
3. Play any MKV file (example: H.264 video, AAC audio).
4. Observe green flicker during playback.
5. Play an MP4 file with the same codec → playback is clean.

System Information:

• OS: Debian GNU/Linux 13 (trixie)
• Parole version: parole --version (include output)
• GStreamer version: gst-inspect-1.0 --version (include output)
• GPU: AMD Radeon RX 6800 (RADV driver)
• Display server: X11
• VAAPI plugins: not present (gst-inspect-1.0 | grep vaapi shows none)
• libav plugin: present (gst-inspect-1.0 libav shows decoders)

Additional Notes:

• MP4 files play fine in Parole.
• MKV files play fine in mpv (ffmpeg-based).
• Flicker persists when forcing sinks (GST_VIDEO_SINK=glimagesink, GST_VIDEO_SINK=ximagesink).
• Suspected regression in GStreamer 1.26.x MKV handling.