Linux: forced WEBKIT_DISABLE_DMABUF_RENDERER=1 makes the main window transparent on WebKitGTK 2.54 (only repainted regions show) #2624

Description

On Linux with WebKitGTK 2.54, Cap's main window is almost entirely invisible. The window exists (KWin reports it mapped, 330×395, not hidden/minimized), but only the regions that get repainted show up: a hovered button, a toggle being animated, etc. Everything else stays transparent, so you see the desktop through the window.

The cause is the unconditional override added for #1948:

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

With WebKitGTK 2.54, disabling the DMABUF renderer on a transparent window makes WebKit repaint only the damaged areas, and the rest of the surface is never painted. Other Tauri projects hit the same regression on 2.54 (e.g. rogeriojunior31/Vults#126, xnmp/tauri-explorer#1057).

Because the variable is overwritten with set_var, users can't opt out: running WEBKIT_DISABLE_DMABUF_RENDERER=0 Cap still yields WEBKIT_DISABLE_DMABUF_RENDERER=1 in the environment of Cap's WebKitWebProcess (checked via /proc/<pid>/environ).

Reproduction

1. Debian 13, KDE Plasma 6.3 Wayland, WebKitGTK 2.54.0.
2. Install Cap 0.6.0 (.deb) and launch it.
3. The main window is transparent except for elements being hovered or animated.

What I tried

All of these still show the bug, because the forced variable stays in effect:

• GDK_BACKEND=x11 (XWayland)
• WEBKIT_DISABLE_COMPOSITING_MODE=1
• WEBKIT_SKIA_ENABLE_CPU_RENDERING=1
• forcing Mesa EGL instead of NVIDIA (__EGL_VENDOR_LIBRARY_FILENAMES=.../50_mesa.json)
• WEBKIT_FORCE_COMPOSITING_MODE=1

Workaround that fixes it: an LD_PRELOAD shim that turns setenv("WEBKIT_DISABLE_DMABUF_RENDERER", …) into a no-op. With the variable unset, the window renders fully and correctly on this machine.

#define _GNU_SOURCE
#include <dlfcn.h>
#include <string.h>
int setenv(const char *name, const char *value, int overwrite) {
    static int (*real)(const char *, const char *, int);
    if (!real) real = dlsym(RTLD_NEXT, "setenv");
    if (name && strcmp(name, "WEBKIT_DISABLE_DMABUF_RENDERER") == 0) return 0;
    return real(name, value, overwrite);
}

Suggested fix

At minimum, respect a value the user already set, so people can opt out without a shim:

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

Better still, don't apply the workaround by default on WebKitGTK ≥ 2.54 (or limit it to the configurations from #1948), since it now breaks rendering instead of fixing it.

Environment

• Cap 0.6.0 (.deb)
• Debian GNU/Linux 13 (trixie), kernel 6.12
• KDE Plasma / KWin 6.3.6, Wayland session
• libwebkit2gtk-4.1 2.54.0
• Hybrid GPU: Intel Raptor Lake UHD (i915) + NVIDIA RTX 4060 Laptop (driver 555.58.02)

Related: #1948, #1553