GTK: background blur extends outside the window geometry

[YaLTeR]

Issue Description

On Wayland, background-blur = true uses the KDE blur protocol on the main Ghostty wl_surface. However, it doesn't set the blur region, meaning that the entire surface gets background blur -- including underneath client-side window shadows, rounded corners, and the fully opaque headerbar.

Expected Behavior

Ghostty sets the blur region to match the terminal area, so only the pixels under the terminal are blurred, and it doesn't leak outside.

This screenshot was made with compositor-side clipping to window geometry (requires manual user configuration).

Actual Behavior

The entire surface is blurred, including outside the terminal area.

Reproduction Steps

1. Open Ghostty.

Ghostty Logs

$ WAYLAND_DEBUG=1 ghostty 2>| grep -E 'blur'
[2876180.668] {Default Queue} wl_registry#2.global(34, "org_kde_kwin_blur_manager", 1)
[2876240.583] {Default Queue} wl_registry#40.global(34, "org_kde_kwin_blur_manager", 1)
[2876240.585] {Default Queue}  -> wl_registry#40.bind(34, "org_kde_kwin_blur_manager", 1, new id [unknown]#42)
[2876363.568] {mesa egl display queue} wl_registry#46.global(34, "org_kde_kwin_blur_manager", 1)
[2876384.038] {Default Queue}  -> org_kde_kwin_blur_manager#42.create(new id org_kde_kwin_blur#50, wl_surface#41)
=========== note how there's no call to org_kde_kwin_blur.set_region(), so the entire surface is blurred
[2876384.047] {Default Queue}  -> org_kde_kwin_blur#50.commit()
[2876534.239] {Default Queue} wl_registry#60.global(34, "org_kde_kwin_blur_manager", 1)
[2878287.193] {Default Queue}  -> org_kde_kwin_blur#50.release()

Ghostty Version

Ghostty 1.2.3-1.fc43

Version
  - version: 1.2.3-1.fc43
  - channel: tip
Build Config
  - Zig version   : 0.14.1
  - build mode    : builtin.OptimizeMode.ReleaseFast
  - app runtime   : apprt.Runtime.gtk
  - font engine   : font.main.Backend.fontconfig_freetype
  - renderer      : renderer.generic.Renderer(renderer.OpenGL)
  - libxev        : io_uring
  - kernel version: 6.18.9-200.fc43.x86_64
  - desktop env   : other
  - GTK version   :
    build         : 4.20.2
    runtime       : 4.20.3
  - libadwaita    : enabled
    build         : 1.8.0
    runtime       : 1.8.4
  - libX11        : enabled
  - libwayland    : enabled

OS Version Information

Fedora 43 Silverblue

(Linux only) Display Server

Wayland

(Linux only) Desktop Environment/Window Manager

niri

Minimal Ghostty Configuration

background-blur = true

Additional Relevant Configuration

No response

I acknowledge that:

• I have reviewed the FAQ and confirm that my issue is NOT among them.
• I have searched the Ghostty repository (both open and closed Discussions and Issues) and confirm this is not a duplicate of an existing issue or discussion.
• I have checked the "Preview" tab on all text fields to ensure that everything looks right, and have wrapped all configuration and code in code blocks with a group of three backticks (```) on separate lines.

[pluiedev]

The behavior under KDE at least is to ignore the CSD borders - I verified this when I was implementing this initially. Note that the KDE blur protocol itself is already sort of ill-defined — have you looked at the new ext-background-effects-v1 protocol and checked if it defines what compositors should do in this case?

[YaLTeR]

I think you should try to shape the region to account for the corners. (Ideally I think this is something GTK itself should do since it knows the widget coordinates and whatnot.) It's a bit scuffed because you have to approximate it with rectangles, but it can work. I'm trying to get various tools and apps to start doing it, here's for example code from the Vicinae launcher, looks alright:

There's also a PR into quickshell.

For it to look best what you want is to make sure you have an opaque border of 1-2 px, and that the region rectangles terminate inside that region, then the jagged edges will be hidden.

[pluiedev]

Cool — it's slightly annoying that the wl_region API doesn't make this easier, but I suppose this would do. Is there a MIT (or compatible) implementation of the corner cut algorithm? We can't really directly copy it from Vicinae as it is GPLv3

[YaLTeR]

Not sure but it isn't really doing anything complicated, just go line by line and remove the right number of pixels from each side

[pluiedev]

I'll try to figure something out then :) Gonna mark this accepted for now. As a Niri user myself, thanks for chiming in and can't wait for the new release!

[YaLTeR]

As a Niri user myself

Hah, suddenly :) You should be able to test with wip/branch then (xray only for now)

[pluiedev]

I've been using Niri for ~half a year now and I'm a big fan :D Will give the WIP branch a try soon

This is now being tracked in #10720 (explicit blur regions) and #10721 (ext-bg-effects support). Thanks again!