Crash when using custom shader with selection-fg/bg options

[rhodes-b]

Issue Description

I was testing a different discussion that was for cursor custom shaders and on startup ghostty crashes.
I tested 1.3.1 and no crash I also built the same commit tip below with ReleaseFast and no crash, so it might exist in 1.3.X debug builds I did not test

Expected Behavior

no crashes

Actual Behavior

Refer to above

Reproduction Steps

I just took the first cursor shader I could find here https://github.com/Linear-optimize/dotfiles/blob/master/ghostty/shaders/cursor_smear.glsl

crashes right away on startup

Ghostty Logs

thread 133335 panic: access of union field 'color' while field 'cell-foreground' is active
/home/user/workspace/ghostty/src/renderer/generic.zig:2073:56: 0x3107774 in updateCustomShaderUniformsFromState (ghostty)
                    @as(f32, @floatFromInt(selection_bg.color.r)) / 255.0,
                                                       ^
/home/user/workspace/ghostty/src/renderer/generic.zig:1418:57: 0x30dadce in updateFrame (ghostty)
                self.updateCustomShaderUniformsFromState();
                                                        ^
/home/user/workspace/ghostty/src/renderer/Thread.zig:610:27: 0x30b4881 in renderCallback (ghostty)
    t.renderer.updateFrame(
                          ^
/home/user/workspace/ghostty/src/renderer/Thread.zig:532:23: 0x30b4787 in wakeupCallback (ghostty)
    _ = renderCallback(t, undefined, undefined, {});
                      ^
/home/user/.cache/zig/p/libxev-0.0.0-86vtc4IcEwCqEYxEYoN_3KXmc6A9VLcm22aVImfvecYs/src/watcher/async.zig:679:38: 0x30db11e in callback (ghostty)
                            return cb(
                                     ^
/home/user/.cache/zig/p/libxev-0.0.0-86vtc4IcEwCqEYxEYoN_3KXmc6A9VLcm22aVImfvecYs/src/backend/io_uring.zig:804:29: 0x3037aee in invoke (ghostty)
        return self.callback(self.userdata, loop, self, result);
                            ^
/home/user/.cache/zig/p/libxev-0.0.0-86vtc4IcEwCqEYxEYoN_3KXmc6A9VLcm22aVImfvecYs/src/backend/io_uring.zig:193:33: 0x303a871 in tick___anon_643270 (ghostty)
                switch (c.invoke(self, cqe.res)) {
                                ^
/home/user/.cache/zig/p/libxev-0.0.0-86vtc4IcEwCqEYxEYoN_3KXmc6A9VLcm22aVImfvecYs/src/backend/io_uring.zig:84:42: 0x303a94a in run (ghostty)
            .until_done => try self.tick_(.until_done),
                                         ^
/home/user/.cache/zig/p/libxev-0.0.0-86vtc4IcEwCqEYxEYoN_3KXmc6A9VLcm22aVImfvecYs/src/dynamic.zig:172:26: 0x303f389 in run (ghostty)
                    ).run(mode),
                         ^
/home/user/workspace/ghostty/src/renderer/Thread.zig:262:26: 0x3079b83 in threadMain_ (ghostty)
    _ = try self.loop.run(.until_done);
                         ^
/home/user/workspace/ghostty/src/renderer/Thread.zig:201:21: 0x3033990 in threadMain (ghostty)
    self.threadMain_() catch |err| {
                    ^
/nix/store/0n9r16gl2a9sh7rdmz1kl3fkc771m8dk-zig-0.15.2/lib/std/Thread.zig:509:13: 0x2feff5a in callFn__anon_516491 (ghostty)
            @call(.auto, f, args);
            ^
/nix/store/0n9r16gl2a9sh7rdmz1kl3fkc771m8dk-zig-0.15.2/lib/std/Thread.zig:781:30: 0x2fa3d73 in entryFn (ghostty)
                return callFn(f, args_ptr.*);
                             ^
???:?:?: 0x7ffff689a979 in ??? (libc.so.6)
Unwind information for `libc.so.6:0x7ffff689a979` was not available, trace may be incomplete

???:?:?: 0x7ffff6922d2b in ??? (libc.so.6)
debug(io_handler): terminal pwd: /home/user/workspace/ghostty
Aborted                    (core dumped) ./zig-out/bin/ghostty --gtk-single-instance=false

Ghostty Version

Ghostty 1.3.2-main-+659019666

Version
  - version: 1.3.2-main-+659019666
  - channel: tip
Build Config
  - Zig version   : 0.15.2
  - build mode    : .Debug
  - app runtime   : .gtk
  - font engine   : .fontconfig_freetype
  - renderer      : renderer.generic.Renderer(renderer.OpenGL)
  - libxev        : io_uring
  - kernel version: 6.12.76
  - desktop env   : other
  - GTK version   :
    build         : 4.20.3
    runtime       : 4.20.3
  - libadwaita    : enabled
    build         : 1.8.3
    runtime       : 1.8.3
  - libX11        : enabled
  - libwayland    : enabled

OS Version Information

Nixos 25.11

(Linux only) Display Server

Wayland

(Linux only) Desktop Environment/Window Manager

sway 1.11

Minimal Ghostty Configuration

selection-foreground=cell-background # note only one of these needs set
selection-background=cell-foreground # and it needs to be a cell-X option
custom-shader = <shader-above>

Additional Relevant Configuration

NA

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.

[rhodes-b]

even loading the shader at runtime after ghostty has initialized by modifying the config leads to the same crash