issue 2021: harden Linux WebKit rendering (fixes black AppImage window)

WebKitGTK's DMA-BUF/EGL renderer fails to init on many Linux GPU/driver/Wayland
setups -> 'EGL_BAD_PARAMETER' -> black window (known WebKitGTK issue, not app code).
Per Tauri's Linux-graphics guidance, set the software-fallback env vars at startup
before the webview is created, scoped to AppImage launches (native installs keep GPU
accel): __NV_DISABLE_EXPLICIT_SYNC / WEBKIT_DISABLE_DMABUF_RENDERER /
WEBKIT_DISABLE_COMPOSITING_MODE, each only if the user already set it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

desktop/src-tauri/src/lib.rs

@@ -10,6 +10,9 @@ mod integration;
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
+    #[cfg(target_os = "linux")]
+    harden_linux_webkit_rendering();
+
     tauri::Builder::default()
         .invoke_handler(tauri::generate_handler![
             integration::integration_status,
@@ -19,3 +22,33 @@ pub fn run() {
         .run(tauri::generate_context!())
         .expect("error while running the ThoughtSync desktop app");
 }
+
+/// WebKitGTK's GPU-accelerated rendering (the DMA-BUF renderer + EGL compositing) fails
+/// to initialize on a wide range of Linux GPU/driver/Wayland setups — "Could not create
+/// default EGL display: EGL_BAD_PARAMETER" → a black/blank window. This is a well-known
+/// WebKitGTK issue that hits Tauri apps broadly, NOT app-specific. Tauri's guidance
+/// (https://v2.tauri.app/develop/debug/linux-graphics/) is to force the software
+/// fallbacks at startup, before the webview is created, so end users don't have to.
+///
+/// We apply the fix ONLY for AppImage launches (where these failures cluster and where
+/// bundled libs are in play), leaving each var overridable — native (.deb) installs use
+/// system libs and keep GPU acceleration untouched. WebKit software rendering is plenty
+/// for this UI.
+#[cfg(target_os = "linux")]
+fn harden_linux_webkit_rendering() {
+    let is_appimage =
+        std::env::var_os("APPIMAGE").is_some() || std::env::var_os("APPDIR").is_some();
+    if !is_appimage {
+        return;
+    }
+    // Ordered per Tauri's escalation ladder; each set only if the user hasn't chosen.
+    for (key, value) in [
+        ("__NV_DISABLE_EXPLICIT_SYNC", "1"),
+        ("WEBKIT_DISABLE_DMABUF_RENDERER", "1"),
+        ("WEBKIT_DISABLE_COMPOSITING_MODE", "1"),
+    ] {
+        if std::env::var_os(key).is_none() {
+            std::env::set_var(key, value);
+        }
+    }
+}