Linux: welcome/onboarding window renders black unless WEBKIT_DISABLE_DMABUF_RENDERER=1 is set

[fnune]

Description

On Linux (KDE Plasma 6 Wayland, AMD GPU) the welcome/onboarding window opens but its body paints as a solid black box, while Cap's own titlebar renders fine. Relaunching with WEBKIT_DISABLE_DMABUF_RENDERER=1 makes the content render correctly, which points at WebKitGTK's DMABUF renderer painting the accelerated content (Cap's Rive/WebGL onboarding) black. Could Cap set that env var on Linux, or is there a cleaner fix?

Reproduction

1. KDE Plasma 6 Wayland session, AMD GPU.
2. Launch Cap. The welcome window body is black.
3. Relaunch with WEBKIT_DISABLE_DMABUF_RENDERER=1. The content renders.

Additional Context

• Cap version: 0.5.2
• OS: CachyOS (Arch), KDE Plasma 6 / KWin Wayland, AMD Hawk Point iGPU (Radeon 780M)
• webkit2gtk 2.52.4, mesa 26.1.2, kernel 7.1.1

[linear]

CAP-741

[tembo]

Linux desktop support was just added in PR #1923 (merged June 18), so this is one of the first Linux-specific rendering issues. The codebase currently has no WebKitGTK environment variable workarounds - searching for WEBKIT or DMABUF returns no hits.

The fix could be implemented in apps/desktop/src-tauri/src/main.rs before the Tauri runtime starts, similar to how RUST_LOG is set for debug builds (line 14). Something like:

#[cfg(target_os = "linux")]
unsafe {
    std::env::set_var("WEBKIT_DISABLE_DMABUF_RENDERER", "1");
}

Alternatively, for a more targeted fix that only affects AMD GPUs on Wayland, you could check for WAYLAND_DISPLAY and parse /proc/driver or lspci output before setting it - though the blanket workaround is simpler and the performance impact of disabling DMABUF is typically minimal for UI rendering.

The onboarding page itself (apps/desktop/src/routes/(window-chrome)/onboarding.tsx) uses CSS animations and image assets rather than WebGL/Rive, so this is purely a WebKitGTK compositor interaction issue rather than anything Cap-specific in the onboarding content.

Related: #1553 (Linux tracking issue), #1946 (another Linux/Wayland issue from the same reporter), #1947 (Linux tray icon theming).