The issue with matching unbalanced braces (picture in previous comment) also happens in unmodified KWrite, so it doesn't seem to be an issue with this revision.
This fails test 49 - kateview_test; but checking out ktexteditor from git and then running make/make install/make test also results in that test failing.
Thus, I believe this revision is ready for review.

Updated patch, closing brackets now get eaten properly in all cases I tested.

I only use Kate/KWrite so didn't test it on the others previously - sorry.
It actually only works in Kate and KWrite, and has no effect on Kile/KDevelop. KDevelop is obviously doing some special stuff, but visually the Kile autocompletion seems identical to the one in Kate/KWrite so I'm surprised that it doesn't work.
Any idea what's causing this issue or where I should look in the KDevelop code?

Removed unnecessary modifications to code

Would it be appropriate to add this as a config option in the settings? Because to me, keeping the sort of whitespace you describe seems like the better behavior:
doSomething(a, b, |
^ is correct. But obviously there are differing opinions on this, and a setting would resolve that.

Forgot to put this in the first submission: it fails test 66 vimode_keys, I believe this may be related to bug 392858 - vi mode :q (quit) command works in Kate but not KWrite https://bugs.kde.org/show_bug.cgi?id=392858
Every other test passes.