We have a hack in the XSETTINGS code to substitute gtk-xft-dpi
with gdk-unscaled-dpi unless the screen has a fixed window scale,
in which case we just use gtk-xft-dpi.

But if the screen is changed to have a fixed window scale, then
the substituted value of gdk-unscaled-dpi will stick around until
the next (coincidental) change to XSETTINGS. To fix this, force
an immediate reread of the XSETTINGS property when
gdk_x11_display_set_window_scale() is used.

https://bugzilla.gnome.org/show_bug.cgi?id=725754