fix(renderer+ux): frame-liveness watchdog (#558), dup-add button (#655)

- #558: a backend can be alive yet wedged (zero presented frames forever
  on legacy stacks). Vendored femtovg counts presented frames; a watchdog
  8s after window-ready re-arms the renderer sentinel and restarts, so
  the existing ladder degrades wgpu -> alt adapter -> GL -> software on
  its own instead of stranding the user on a blank window.
- #655: 'Add only new tracks' stays enabled with all-dupes (adds zero
  and closes) instead of disappearing as the primary action.

km

crates/vendor/i-slint-renderer-femtovg-1.16.1/lib.rs

@@ -67,6 +67,26 @@ impl core::fmt::Display for FrameSkipped {
 
 impl std::error::Error for FrameSkipped {}
 
+/// QBZ vendor patch (issue #558): presented-frames counter feeding the
+/// render-liveness watchdog in the qbz crate. A backend can be ALIVE yet
+/// wedged — the #558 Outdated-surface skip loop above skips every frame
+/// forever while the process looks healthy, and the crash-chain ladder only
+/// escalates on DEATH, so a silent wedge never degrades. The watchdog reads
+/// this a few seconds after startup: zero presented frames with a visible
+/// window = escalate the degradation ladder and restart on the next rung.
+static FRAMES_PRESENTED: core::sync::atomic::AtomicU64 = core::sync::atomic::AtomicU64::new(0);
+
+/// Total frames successfully presented by the active backend since process
+/// start. Read by the qbz render-liveness watchdog (#558).
+pub fn frames_presented() -> u64 {
+    FRAMES_PRESENTED.load(core::sync::atomic::Ordering::Relaxed)
+}
+
+/// Watchdog hook — each backend's `present_surface` calls this on success.
+pub(crate) fn note_frame_presented() {
+    FRAMES_PRESENTED.fetch_add(1, core::sync::atomic::Ordering::Relaxed);
+}
+
 pub trait GraphicsBackend {
     type Renderer: femtovg::Renderer + TextureImporter;
     type WindowSurface: WindowSurface<Self::Renderer>;

crates/vendor/i-slint-renderer-femtovg-1.16.1/opengl.rs

@@ -196,7 +196,13 @@ impl GraphicsBackend for OpenGLBackend {
         &self,
         _surface: GLWindowSurface,
     ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
-        self.opengl_context.borrow().swap_buffers()
+        let result = self.opengl_context.borrow().swap_buffers();
+        if result.is_ok() {
+            // QBZ vendor patch (#558): count presented frames for the
+            // render-liveness watchdog.
+            crate::note_frame_presented();
+        }
+        result
     }
 
     #[cfg(not(target_family = "wasm"))]

crates/vendor/i-slint-renderer-femtovg-1.16.1/wgpu.rs

@@ -209,6 +209,9 @@ impl GraphicsBackend for WGPUBackend {
         surface: Self::WindowSurface,
     ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
         surface.surface_texture.present();
+        // QBZ vendor patch (#558): count presented frames for the
+        // render-liveness watchdog.
+        crate::note_frame_presented();
         Ok(())
     }