I'm using the filter bar without a modifier key, so I can filter directly typing in.
Just now I discovered when ESC'ing out of the filter bar, that does not clear the filter box (inserted search characters) but the first character stays in the filter field. Hitting ESC a second time clears the filter field completely.
Is this possibly bug or is it a compromise for having the filter bar react without modifier key?
That's by design. The first Esc lets you give focus back to the file display, the second will clear the filter. You don't always want to do both at once.
But the first ESC on my setup clears all but the first characters from the filter bar - lets say I search for "Meowski", my first ESC deletes anything but the "M". Is that also by design?
Oh yeah, the first Esc also resets the filter to what it was when you last activated it, so you can undo the last changes you made. (You can use Return instead to keep the changes and give focus back to the file display.)
This is a bit odd, as in unexpected. I initially thought ESC would entirely reset the filter bar. But no matter how long or short my search word is, ESC leaves the first character in it. I can live with it fine, I only dont understand the use of it 
I see what you mean now, with the activation key turned off. (Before I assumed the first character was from a previous use of the filter bar.) That is unintentional, and something we should be able to fix.
Aaaah, that sounds great, many thanks Leo!