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Bold font issue

I am having an issue with bold fonts in the newly upgraded system, which is KDE Frameworks 5.37.0 using Qt 5.9.1, running on NixOS 17.09.

I expect bold fonts on window title displayed as below:

This is FreeSerif bold. But I found that this could only be achieved if the "General" font was set to bold, like this:

This means almost everything else are all displayed in bold font as well, which is unacceptable.

If only "Window title" is set to bold, and "General" is set to regular, the font on the window title will be "simulated" based on the regular style (sorry for I do not know the right way to talk about this), as below:

As you can see, those characters are actually not bold.

Is this a known issue? How to fix it?

Thanks in advance.

Best regards,
Guanpeng Xu