bug#50440: Emacs does not rescale fonts when display changes DPI

[Eli Zaretskii]

> From: Radon Rosborough <radon.neon@gmail.com>
> Date: Mon, 6 Sep 2021 13:04:27 -0700
> 
> I run Emacs either on my laptop (1920x1080, 96 DPI) or on my external monitor 
> (3840x2160, 192 DPI,
> configured with 200% display scaling so that applications display identically 
> on both displays, and with my
> laptop display disabled).
> 
> What I observe is that if I start Emacs on my external monitor and unplug it 
> (so the display DPI decreases
> from 192 to 96), the fonts in Emacs appear to be twice as large, although the 
> window title bar is sized
> normally and all other applications scale correctly. Similarly, when I start 
> Emacs on my laptop screen and
> then plug in my monitor, the fonts in Emacs appear to be twice as small, 
> although all other applications scale
> correctly.
> 
> I observed that when I start Emacs on my external monitor, M-x list-fontsets 
> returns "-CTDB-Fira
> Mono-normal-normal-normal-*-29-*-*-*-m-0-fontset-startup", whereas when I 
> start it on my laptop screen, it
> returns "-CTDB-Fira 
> Mono-normal-normal-normal-*-15-*-*-*-m-0-fontset-startup". The fontset does 
> not
> change when the underlying display DPI changes, which is presumably part of 
> the issue.
> 
> By inserting some debugging code into Emacs, I saw that x_term_init is only 
> called once, when Emacs
> initially starts up. This appears to be where the DPI is set in the 
> x_display_info struct, and then it is never
> updated again. I tried manually invoking x-open-connection from Lisp after 
> switching displays, which did call
> x_display_info again and set a new DPI value. However, this value didn't seem 
> to propagate to
> FRAME_RES_Y which is called from font_pixel_size when initializing the fonts, 
> for some reason. Moreover,
> it seems like additional changes would be required to get the default font to 
> be reinitialized even if the
> underlying x_display_info is updated.

We call x_term_init each time a frame is created for the first time on
a new display.  So potentially make-frame-on-display could call it.

One other thing to try is to start Emacs as a daemon, and when you
switch displays, close all the client frames and make new ones via
emacsclient.

Does any of the above allow you to solve the issue?