I have the same problem, the characters overlaps the own space.
But, I have the problem only on qt5 interface
When the ide is compiled on gtk2, the problem disappear
AFAIK synedit only acceps monospaced fonts.Well yes... and a yet, not.
So in that sense, SynEdit will "work" with none monospaced fonts, but the result is usually real bad.
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In the QT example, SynEdit got good measurements, but the real output is actually wider. No idea why.
Looking at proportional fonts with a monospaced distance is unreadable, makes headache and should be prohibited by the IDE
The bug is, that the IDE allows also proportional fonts.
I think isn't a font problem, look this images, one is gtk2 and other is qt5(with fail).Well, what I can see is that while you chosen the same font (font of the same name) under gtk2 and qt, the font that is used is different.