1/2/4 looked like powers of 2, i.e. a flag set.
Using 5 here gives is the same value as FixedFontsOnly | ShowDifferences.

I missed the fact that combining two or more flags would give the same value... will fix.

On the other hand it makes sense FixedFontsOnly would automatically hide the checkbox, no? If the app says "fixed fonts only", the user has no choice in the matter.

Yeah, that makes sense (a special case is Konsole, which embeds KFontChooser and adds a checkbox to "show all fonts", IIRC this is to offer users a way out with some mis-configured fonts... etc).

I'm also confused by the name "No[...]Toggle" and the docu saying this will show a checkbox. Something's off by "not" ;)
Sounds wrong?

I wrote the enum docs, _then_ changed the name of the member to NoFixedOnlyToggle (this way apps can set it if they want but it doesn't have to be set by default, i.e. we always show the box unless told otherwise). So yeah off by "Do not".

maybe document that FixedFontsOnly implies NoFixedCheckBox?

(as in, the widget will behave as if NoFixedCheckBox was set)

I would add a final ",", so you do not need to touch the line if any further flags are added in the future. Improves git blame display per line :)

Yes, will do (less surprises for users of the class).

Wouldn't that look a bit weird as if something has been removed or missing?

I might perhaps while getting used to if you are coming from old code habits, but the syntax of C++ has been extra extended to allow this, among others to make patches/diffs less noisy or #if#else around list ends, but many also fancy this for line-by-line git blame improvements.

And the majority of projects I contribute to now does this by default, and no confusion known to me, the eyes get quickly adapted to the pattern, even more as each line now has the same content