Window maximizes at first but won't after moving window

[vidavi]

I'm on Big Sur, and when I first ran a video, VLC maximized the window on the screen (so that there were letterbox and pillarbox bars, because the video resolution was smaller than the screen, as in, it didn't scale the video to the viewport), but when I went to move the window to another screen, it shrank the window vertically (so the pillerbox bars went away), but not horizontally (so the letterbox bars were still there, and the window was maximized horizontally), and nothing I did could get the window maximized vertically again... like double-clicking the title bar, or dragging it up.

This is so strange. So what's irritating about a one-axis maximized window is that I may accidentally click something above or below the window and then the whole window goes away; I want the window maximized vertically and horizontally regardless of how big the actual video resolution is. I'm OK with letterbox and pillarbox bars. At that point I should be able to choose whether to scale up the video inside of the window at the risk of blur...

Right now I just can't figure out how to get the maximized window back (regardless of the video resolution). Double-clicking the title bar of the player just maximizes the window horizontally. I have this issue in Quicktime as well, so I can't tell if it's a macOS thing or I'm unaware of a VLC setting.