Pertains to pavucontrol 2.0-2 from Arch repo:
After starting pavucontrol, I typically resize it to the minimum horizontal width that it accepts (which seems to be 666 pixels on my system). Thereafter, several times per hour, pavucontrol autonomously resizes itself horizontally to 917 pixels (which appears to be the "default" width, i.e. the width it comes up in upon starting.)
When this happens, I shrink it back to 666... but a little while later -- minutes usually, but sometimes hours -- ... it resizes back to 917.
Have not yet been able to identify any particular event that causes the resizing. But it seems to have nothing to do with any mouse input (or any other user action). It just happens "by itself".
This is Royal PITA.
As of pavucontrol 2.0 the window is automatically increasing if long text lines have to be shown but doesn't decrease once the text has gone.
This could very well have been the reason of the problem you're facing: E. g. some lengthy track title may have made the window increase, but right when you were looking another track featuring a smaller name or just nothing was played and hence you couldn't see what had happened.
That way pavucontrol windows may even increase to a size exceeding the actual display, see https://bugs.freedesktop.org/show_bug.cgi?id=42242. (If you should want to have a look at the fix note it's available via VCS only right now.)
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