Qt does not set the WM_NAME property for window titles, only _NET_WM_NAME.
It seems it's not strictly required according to ICCCM/[EWMH|http://standards.freedesktop.org/wm-spec/wm-spec-1.3.html#idm140130317621408]:
The WM_NAME property is an uninterpreted string that the client wants the window manager to display in association with the window
The Client SHOULD set this to the title of the window in UTF-8 encoding. If set, the Window Manager should use this in preference to WM_NAME.
Qt 5's XCB backend doesn't support many "deprecated" features where there is a
EWMH replacement. For example it also doesn't support setting a window icon
through the WM_HINTS (ICCCM section 184.108.40.206) property. Given that I
interpreted this as a design decision to not support the "deprecated" hints in
the new implementation.
However, unfortunately not setting WM_NAME causes problems in the wild, for example:
- herbstluftwm (a window manager) not updating titles correctly.
- KeePassX (a password manager) not reading the titles correctly.
- Probably anything not implementing EWMH not reading the title correctly, which might affect some other minimal or older WMs.
I understand the argument of not maintaining "deprecated" things, but here it seems it really causes trouble at a lot of other places - of course I'll submit bug reports there as applicable as well.