It's interesting to me to make Qt Quick (controls) to be compiled with QT_QUICK_DEFAULT_TEXT_RENDER_TYPE=NativeRendering which should improve text kerning in some qml controls. That's the strange distance between letters in qml (not in any C++ widgets).
@demm did you plan to do it, i can test locally compiled qml to test it, if you're interested?

Hmm, that would explain...
In my development of a whole new Locale module for Calamares, I see constant issues with rendered text. So much difference between running code in qmlscene, running calamares with sudo or kdesu. Would that help?

You can test with export QML_DISABLE_DISTANCEFIELD=1 also

11 days later

The patch is incorrect to me it should be

x = qRound(itemWidth / 2.0f - textWidth / 2.0f);

Can you try it like this, official one isn't working correct to me.

I'll notify in bug as well, new patch is working correctly to me, @demm can you confirm?
Especially in Folder View and Audio applet tabs.

It's confirmed that it's incorrect you can try above one or

x = qRound(itemWidth / 2.0f) - qRound(textWidth / 2.0f);

I will remove patched qt5-declarative from the build repo, and wait for Qt 5.15/5.15.1 for a proper, upstream approved fix. It is not like this is something new, so no rush now with unproven and unapproved patches.