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When vlc is launched from the system menu with no video, its window occupies the full screen width and can't be resized horizontally: no grips appear, not even when the window is moved sideways to expose a side border. Grips appear for the top and bottom borders. When it's launched by association by double-clicking a video, likewise: regardless of the dimensions of the video, vlc centers it in a full-width window that can't be horizontally resized, and has black empty space in the client area symmetrically on both sides of the video.

A specific case: a video I just recoded with Handbrake, where mktoolnix reports

Pixel width 1280
Pixel height 720
Display width 957
Display height 400

"Resize interface to video size" is set.

VLC 3.0.17.4 on Linux Manjaro, kernel 5.15.78-1, KDE Plasma 5.26.3, KDE Frameworks 5.99.0, QT 5.15.7

To elaborate: if I manually delete ~/.config/vlc/vlc-qt-interface.conf, vlc's behavior returns to normal, but the problem of abnormal windows always returns. With a hint of exactly where to find the code that writes this config file, I might be able to understand its coded contents.

Well the Qt UI code is in modules/gui/qt/ if that's your question

I've narrowed this down to a specific, repeatable example: a video file which, when I play it on this system with vlc, always causes vlc to play later media -- audio and video -- in a full-screen-width window that can't be narrowed: the side edges can't be grabbed. Videos are their normal height and width but centered horizontally between black horizontal borders. The culprit video began life as MP4, where it causes this bad behavior with later media; repackaged as Matroska using mkvtoolnix it also causes the bad behavior.

Perhaps the bad behavior seen with audio files may shed some light. When I reset vlc's config and play an audio with vlc, it displays the (black) content area at the size of the last use. After playing the evil video, when I play the same audio the window is the same size as for the video, and can't be narrowed.

I have screen grabs of everything and mkvinfo output for the culprit video.

This doesn't happen with mpv, so it really appears to me to lie within vlc. What else can I do to help solve this persistent problem?

To be honest, at this point the only things that would really help are either a detailed analysis of what/where it goes wrong in the code, or a patch.

The few developers working on the Qt UI are busy with VLC 4, and unlikely to have time for this.

Perhaps this topic needs a new title, because I can now characterize the behavior better.

If I play a video that occupies the full screen width, then after that vlc always plays videos, even narrower videos, at their proper height but centered horizontally in a full-width window that can't be resized horizontally. If I reset vlcrc, all videos now play at their correct widths until after the next time I play a full-width video. This behavior doesn't happen on Windows.

Even if this behavior lies somehow in Qt, it seems to me that vlc must be doing something to make it happen, and if that area is now in active development, now is a good time time to track it down and squash it.

In active development? If you are using VLC 4.0, then yeah, and by all means file a bug.

If you are using VLC 3.0, the GUI is in anything but active development. There are precisely zero people working on it. Accordingly unless someone volunteers to patch the bug out, it'll just stay there until/unless 4.0 comes out, unfortunately.