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

[Eli Zaretskii <eliz@gnu.org>]

> From: Clément Pit-Claudel <cpitclaudel@gmail.com>
> Date: Wed, 20 Mar 2019 10:55:43 -0400
> 
> 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?

That'd be very inelegant, IMO.

Instead, I think when a frame is created, we should record its
high-DPI state in the frame structure, or maybe in the frame's
parameters, and then use that when we prepare the fringe bitmaps for
display.

> 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?

If a frame can be moved from high-DPI to low-DPI, then I guess we will
need to query the low-level interfaces which report that in
update_frame or thereabouts.