What is the proper way to scale fringe-bitmaps for high-DPI displays?

[Clément Pit-Claudel]

Hi all,

Users on Flycheck are complaining about poor readability of our fringe bitmaps 
on high-DPI monitors, as the bitmaps look tiny on such screens.  An easy fix is 
to double the size of the bitmap, but it leaves users of low-DPI monitors in 
the cold.  A trickier fix would be to dynamically detect the current monitor's 
density, and pick the appropriate bitmap accordingly, but I'm not entirely sure 
how to do detect these high-DPI monitors:

- x-display-monitor-attributes-list seems OK, but looks more complex than what 
we need (based on looking at the C code) — is it OK to call it repeatedly to 
figure out the current monitor's density for a given frame?

- x-display-pixel-width and x-display-pixel-mm seem simpler, but the 
documentation says 'On "multi-monitor" setups this refers to the pixel width 
for all
physical monitors associated with TERMINAL.'.  What does this mean?

Also, how do applications typically deal with frames being moved from a low-DPI 
monitor to a high-DPI one? Is that an issue in practice?

Thanks!
Clément.