Context for translators:

"eventful" is to show only the items in the logger for which uBO actually did something: blocked, allowed, redirected, hid, etc.
Anyone seeing this?

Input shrinks in width when I start typing and logger content changes, clicking on "V" restores it back.
Looks fine on my end.

Firefox only?
First screenshot is with chromium, it also works fine with Firefox stable on my end.

I see this on all branches of Firefox. Left and right borders of the input switches between 8 and 2.
OK: top/bottom - 0; left/right - 8
Bad: top/bottom - 2; left/right - 2
When typing, after short delay, width changes temporarily from 240 to 252 and then back. Something with "Toggle filtering" button status (color?)?
I've not seen this behavior on my side. Does this happen in a new profile?
Yes
Ok I can reproduce some changes in width, though not as pronounced as what you report. Now I can investigate.
It's related to the background color set by uBO, somehow. No need to type characters in the field, what matters is that the input field's focus is toggled:
input field:focus on itbackground-color property on uBO's rule input:focusYou will see a change in the size of the input field depending on whether the background color property is that of uBO or the user agent's default one.
Forcing a default background color to something fixes the issue. Alternatively, removing the rule input:focus also fixes the issue.
Most helpful comment
It's related to the background color set by uBO, somehow. No need to type characters in the field, what matters is that the input field's focus is toggled:
inputfield:focuson itbackground-colorproperty on uBO's ruleinput:focusYou will see a change in the size of the input field depending on whether the background color property is that of uBO or the user agent's default one.
Forcing a default background color to something fixes the issue. Alternatively, removing the rule
input:focusalso fixes the issue.