On Linux, Firefox is listening on notify::scale-factor to detect DPI
change. However, scale-factor is an int and on the lower-end side of
the DPI scale, some devices are using fractional scale factors encoded
into Xft/DPI setting. Changing from ×1 to ×1.5 scale is therefore
undetected.
The proposed change is two-folds:
- remove use of a cached sDPI value and rely on GTK being the cache
- listening on notify::gtk-xft-dpi to trigger a DPI change
What is missing:
- performance evaluation of not caching sDPI (on a 10s session
loading 2 pages, there is an "overhead" of 6ms on my setup, nothing
visible from my point of view)
- when changing Xft/DPI and scale, the change is done twice, this
seems harmless
Differential Revision: https://phabricator.services.mozilla.com/D92095
10ae6bfc55