Creating a QFont uses faux bold and italic, instead of intended font

If I create a QFont and initialize it via fromString() or setFamily+setBold+setItalic, then widget.setFont(font), then the widgets renders the "regular" font file, with faux bold/italic applied if those attributes are true.

The most obvious errors:

• "light" fonts (eg. Source Sans Pro Light) are not thin.
• "italic" (eg. Lato Italic) is faux-oblique instead.
• "bold" fonts are blurry (less obvious).

If I pass the QFont object through QFontDialog::getFont() and click OK without any modifications, any widgets using the font use the "real" font file.

The errors are fixed:

• "light" fonts are thin.
• "italic" is actually italic.
• "bold" fonts are not blurry.

I observed this bug systemwide in Kubuntu:

• KDE lock screen uses faux oblique when displaying caps-lock or wrong-password
• Dolphin uses faux oblique for symlinks

I also observed in PyQt. I do not have a C++ test program at the moment.

In PyQt from Pip:

• QTableView "selected headers" use faux bold (Fusion style).
• Calling QFontDialog once on a font populated by fromString() or setFamily() returns a inequal font.
  • Calling QFontDialog on a QFontDialog-obtained font returns an equal font. QFontDialog is idempotent.
• If I pass a QFont (not fromString()) to a QFontDialog, it will mistakenly highlight the Oblique option instead of Book.
  • If I open a font dialog in KDE or PyQt, and switch from DejaVu Math TeX Gyre (regular only) to DejaVu Sans, I also get Oblique.
  • If I pass a QFont from a QFontDialog into a second QFontDialog, it highlights the correct Book option.

 This bug seems similar to https://bugreports.qt.io/browse/QTBUG-43400 from years ago. That bug didn't mention that QFontDialog returns a non-broken QFont.