Individual GNOME applications
Individual GNOME applications
Posted Jun 25, 2025 13:29 UTC (Wed) by farnz (subscriber, #17727)In reply to: Individual GNOME applications by dskoll
Parent article: GNOME deepens systemd dependencies
If you have different scale factors for each screen, so that windows appear the same size as they cross the boundary even though the screens have different native PPI, you'll see the issue on X11, but not Wayland. You won't see this with 4 monitors of same diagonal size and resolution.
Under X11, the display is represented as a single large buffer, rectangular sections of which are sent to scanout. Scaling is applied to content as the window content goes from the application's internal representation to the buffer, and this scale factor is the same for all monitors (whether that's the 200 PPI laptop display I'm using as a second screen, or the 150 PPI external monitor I use as my primary display). As a result, I cannot set a scale factor that's right for both monitors - there will always be a discontinuity at the seam, because X11 assumes that something 1,000 pixels high on my laptop screen is visually the same height as something 1,000 pixels high on my external screen, and that's not true (on my laptop screen, that's about 5 inches, on my external screen, it's a bit over 6.6 inches).
Wayland's architecture works differently; the display is considered as-if it's composited from individual windows at scan-out time. The compositor and the application negotiate the window's scaling factor, so the application knows that it's rendering something that's visually meant to be 1,000 abstract units high, but that it's being expected to render (say) a 2,000 pixel high buffer that will be scaled down by the compositor. The compositor can then scale that down to 1,500 pixels on my external monitor (10 inches), and display it at 1:1 on my laptop screen (10 inches), making it look the same on both monitors.
Wayland can also renegotiate the scaling as windows move; so when a window is on my external monitor only, the application can know that it'll be rendering a 1,500 pixel high buffer, which will be rendered as-if it filled 1,000 abstract units of space, while moving that window to my laptop screen gets the same application told to render to a 2,000 pixel high buffer, which will be rendered as-if it filled 1,000 abstract units.