The item Q15709419 was merged to Q3058198. I set with
https://www.wikidata.org/wiki/Special:SetAliases/Q3058198/de the alias " " (U+202F NARROW NO-BREAK SPACE) again. The problem is still the same.

Not allowed characters are ok, but it should be the same characters everywhere.

Screenshot of https://www.wikidata.org/wiki/Q3058198?uselang=de

Screenshot of https://www.wikidata.org/wiki/Q3058198?uselang=de

The alias with U+202F NARROW NO-BREAK SPACE is still not editable or removable.

Attached:

It no longer blocks the interface, but also it's not changeable... that's probably because:

22:07:27.510 $.trim( ' ' /* U+202F NARROW NO-BREAK SPACE */ ) === ''
22:07:27.516 true

We should probably use a trim function in JS and in PHP which do exactly the same (that might mean making NARROW NO-BREAK SPACE not usable as label/ alias).

Changing this bug, because it's only a result of invalid data... you shouldn't have been able to set that alias using the special page in the first place.

Set U+202F as alias: https://www.wikidata.org/wiki/?diff=247934139, the GUI still works.

In T62828#1611276, @Ricordisamoa wrote:

Set U+202F as alias: https://www.wikidata.org/wiki/?diff=247934139, the GUI still works.

The GUI still does not allow to edit or remove this alias:

for reference it is this item: https://www.wikidata.org/wiki/Q15397819

In T62828#3174922, @Fomafix wrote:

I can restore the U+202F and it is not possible to remove or edit this value with the GUI.

Sure, so just don't restore it. Restoring old revision is (partly) bypassing validation, that is known and expected here (yet, probably doesn't happen in the real world).