It can be observed also on Polish Wikisource, but only with vector and modern themes.
No this problem in pl.ws with monobook, Cologne Blue or minerva. Also no this problem in Wikipedia and some other Wikisources (fr, it, bn), with the same MW version installed (1.31.0-wmf.17 (rMWdfc8ff36c673))

May it be some default gadget related?

In T186442#3944930, @Ankry wrote:

It can be observed also on Polish Wikisource, but only with vector and modern themes.
No this problem in pl.ws with monobook, Cologne Blue or minerva. Also no this problem in Wikipedia and some other Wikisources (fr, it, bn), with the same MW version installed (1.31.0-wmf.17 (rMWdfc8ff36c673))

May it be some default gadget related?

OK. Found it. In pl.ws it was related to extra styling defined in [[MediaWiki:Vector.css]]

@gh87 The narrower (width:80% edit summary input field) and character counts pushed outside it is likely caused by the line "width: 80%;" in
https://en.wikisource.org/wiki/MediaWiki:Gadget-enws-tweaks.css
(section input#wpSummary)

And the checkbox indentation is caused by "padding-left: 4.75em;" in the
div.editCheckboxes section of the same CSS file.

So it seems to be local wiki CSS problem.

The above CSS lines should be reviewed by an en.ws admin (first one either removed or completed with appropriate styling for the counter; unsure about the second)

Fixed in https://en.wikisource.org/w/index.php?title=MediaWiki%3AGadget-enws-tweaks.css&type=revision&diff=7333081&oldid=6308601

Thanks, @Esanders . Can the status be changed to "Resolved" or "Open"?

@gh87: Invalid status is fine for problems created by local on-wiki content.