Tried it with just one file - result stays the same on my machine.
So now I would recommend that you investigate your local environment and which program or piece of hardware may interfere.
Could it be a broken keyboard?
Is there some kind of keyboard assistance program that helps to input capitals, e.g. the native Windows program to help handicapped persons.
If you have the chance to try the same thing on a different computer: does it also happen there?
My keyboard is fine for sure. I turned off hotkeyboard when tested it.
I tried it on Windows 8.1 (on virtual machine) and problem does not appear. But on VM audio player runs too slow, while on my Windows 7 it runs almost immediately
Hm, I've tried it out about 50 more times and this problem doesn't appear every time.
Well, it seems like to reproduce the issue "special timing" is required. I've tried another 50 times and I made it appear only 5-7 times. When I press "Shift" in mp3tag, issue disappears
Okay I think maybe my keyboard jams randomly... Sorry for wasted time =(
This is just phantasizing:
Could it be that the data handover to the player has not been concluded when you click on the files list in MP3tag again?
Could you assign (temporarily) a different player as default player and see if it is still like that?
Data seems to be fully loaded when Aimp is shown.
I've downloaded Winamp, Kmplayer and set it as default player. But so far I can't force it show on program start, so Mp3tag doesn't loose the focus.
Well it is not my keyboard 100%. I wrote a program, that shows whether "Shift" or "Ctrl" is pressed. When bug is happening, none of them are pressed
One more observation. I assume, that bug appears only when Aimp is attached to right-side of monitor (see on screenshot #1). When amimp is detached (screenshot #2) bug does not appear.
So I would still conclude that you have a problem with aimp and not with MP3tag. MP3tag is only the messenger that aimp messes with the keyboard input.
But thank you for being so eager to find the circumstances.
No. I have my Winamp docked at the top of the screen and with "always on top" option, so that I can see the tags all the time [unless I make some software full screen]