For some reason the font size Notepad++ uses to render tab fonts suddenly became huge:
There are settings for it in
Settings -> Style Configurator -> Global Styles -> Active tab text and
Inactive tabs, but for some reason changing this setting has no effect. There is nothing suggesting that this setting or any related changed inside Notepad++ (and I sure didn't do it deliberately). Perhaps the IT department pushed out some new Windows 10 font setting (not otherwise visible in the system)? If so: how can I tell what setting to revert? Everything I've found on Windows 10 Professional's font settings has a size of 9, so it seems like a long-shot.
Edit: I just reinstalled Notepad++, after having wiped all npp data, but without any visual effect (the font size in the tabs is still huge). I still suspect our IT department to have pushed some Windows 10 font setting.