I took a quick look at jquery.byteLimit.js which implements the byte-length input limiting... it seems the cursor being moved is a side effect of modifying the contents of the input field (the trimming).

Note the same thing happens if you *type* past the limit too and aren't at the end already -- by default if you happen to be typing all-ASCII-able text you may not notice because you're hitting the browser's character limit.

One could possibly jump through some hoops to get the cursor position and reset it, but I wonder if the whole thing's worth it...

It might be better to give a warning that you're out of characters and just show a red border or a warning text, and let it just crop on the backend.

(In reply to Brion Vibber from comment #1)

One could possibly jump through some hoops to get the cursor position and
reset it, but I wonder if the whole thing's worth it...

There aren't that many hoops to jump through, at leats if you're willing to ignore IE<=8. I say that we are willing and will submit a patch in a second. :)

Change 114959 had a related patch set uploaded by Bartosz Dziewoński:
jquery.byteLimit: Restore caret position after limiting

https://gerrit.wikimedia.org/r/114959

Change 114959 abandoned by Bartosz Dziewoński:
jquery.byteLimit: Restore caret position after limiting

Reason:
I don't remember how this code works anymore. Meh.

https://gerrit.wikimedia.org/r/114959