Patches for Observable Drag Detection / Updates

Sources/Backend/GTK4/Rendering/GTKViewHost.swift

@@ -236,19 +236,46 @@ public class GTKViewHost: AnyViewHost, DependencyTrackingHost {
  observationDidFire = false
  lock.unlock()
 
- // --- Narrow mutation path: try text/color in-place update ---
- // Skipped when withObservationTracking's onChange fired — the narrow
- // path returns without re-running body under withObservationTracking,
- // which would leave @Observable subscriptions dead after the first
- // change. Fall through to the full rebuild so observation re-registers.
- if !fromObservation,
- let describeBody = describeBody,
+ // --- Narrow mutation path: try text/color/canvas in-place update ---
+ // Applied for both @State- and @Observable-driven changes. A full
+ // rebuild tears down the widget tree, which cancels any in-flight
+ // gesture (e.g. a drag on a Canvas knob); the narrow path mutates in
+ // place and preserves it. For an @Observable change the describe pass
+ // below is run under `withObservationTracking`, so re-reading the
+ // observed properties re-registers the one-shot subscription — the same
+ // re-registration the full rebuild gets from `buildBodyWithTracking`,
+ // without the teardown. If the change is not narrow-applicable we fall
+ // through to the full rebuild, which re-registers observation as before.
+ if let describeBody = describeBody,
  let oldRetained = lastRetainedDescriptor,
  let oldExecutor = retainedExecutor {
 
  let previousEnv = getCurrentEnvironment()
  installRebuildEnvironment()
- let described = gtkDescribeCapturingCanvasPayloads(describeBody)
+ let described: (descriptor: GTK4DescriptorNode, canvasPayloads: [GTK4CanvasPayload])
+ if fromObservation {
+ #if canImport(Observation)
+ if #available(macOS 14.0, iOS 17.0, *) {
+ var captured: (descriptor: GTK4DescriptorNode, canvasPayloads: [GTK4CanvasPayload])!
+ withObservationTracking {
+ captured = gtkDescribeCapturingCanvasPayloads(describeBody)
+ } onChange: { [weak self] in
+ guard let self else { return }
+ self.lock.lock()
+ self.observationDidFire = true
+ self.lock.unlock()
+ self.scheduleRebuild()
+ }
+ described = captured
+ } else {
+ described = gtkDescribeCapturingCanvasPayloads(describeBody)
+ }
+ #else
+ described = gtkDescribeCapturingCanvasPayloads(describeBody)
+ #endif
+ } else {
+ described = gtkDescribeCapturingCanvasPayloads(describeBody)
+ }
  setCurrentEnvironment(previousEnv)
 
  let newIdentified = gtkIdentifyDescriptorTree(described.descriptor)