Font don't match resolution anymore – why?

["Felix E. Klee" <felix klee inka de>]

After a recent update, certain X applications, at least those based on
GTK+ 3 don't respect the DPI setting anymore:

    $ xdpyinfo | grep -B2 resolution
    screen #0:
      dimensions:    2880x1620 pixels (381x214 millimeters)
      resolution:    192x192 dots per inch

The result is tiny fonts, for example in the EMACS menu bar.

*Is that an issue with X or with GTK+? How do I get GTK+ 3 applications
respect the DPI settings?*

I know that one can scale GTK+ 3 UI elements by adjusting the
environment variables `GDK_SCALE` and `GDK_DPI_SCALE`. However, for
getting correct font size that should not be necessary, at least in the
past it was not necessary.