Summary:
When Minecraft is launched from the NEW launcher, trying to resize the window makes the GUI glitch out.

NOTE:
It has to do with redrawing: When placing the mouse over a button (at the location it should be normally), it will highlight!

NOTE 2:
This affects ALL MC versions: I tested a version (before the new launcher) I used last without problems, this time the bug

Screenshot 1 - Happens when scaling down
Screenshot 2 - Happens when scaling up

There is a way to refesh it:
Click out of the window (as to make it unfocussed) and then click back in. After a couple of seconds, it will redraw correcly

I confirmed this only happens in the new launcher:
Downloaded the latest older version (Bootstrap v5) and launched the save MC version -> Resize works
Also when modding in Eclipse, the version launched from Eclipse resizes seemlessly