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

["Clément Pit-Claudel" <cpitclaudel@gmail.com>]

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.