[SDK] Keep a value that reaches reconcile as its loop winds down

The loop that drives `scope.reconcile` was marked as running until the
promise of its last run had settled, which is a few microtasks after the
loop itself had checked for new values. A value arriving in that window
found the loop "running", was left to it, and was never processed: a
connection appearing at that moment would never get its publisher.

The flag is now cleared the moment the loop ends, so such a value starts
a new loop. The regression test drives the window microtask by microtask.

sdk/src/reactive/ObservableScope.test.ts

@@ -192,6 +192,27 @@ describe("Reconcile", () => {
     expect(setup).not.toHaveBeenCalledWith(3);
   });
 
+  // The loop ends a few microtasks before its promise settles; a value
+  // arriving in that window used to be dropped
+  it.each([0, 1, 2, 3, 4, 5, 6])(
+    "picks up a value that arrives %i microtasks after the previous loop ended",
+    async (ticks) => {
+      const scope = testScope();
+      const behavior$ = new BehaviorSubject<number>(0);
+      const setup = vi.fn();
+      scope.reconcile(behavior$, async (value) => {
+        setup(value);
+        return Promise.resolve();
+      });
+
+      for (let i = 0; i < ticks; i++) await Promise.resolve();
+      behavior$.next(1);
+      await vi.advanceTimersByTimeAsync(10);
+
+      expect(setup).toHaveBeenCalledWith(1);
+    },
+  );
+
   it("should wait for setup to complete before starting cleanup", async () => {
     vi.useFakeTimers();
     const scope = testScope();

sdk/src/reactive/ObservableScope.ts

@@ -165,13 +165,15 @@ export class ObservableScope {
     callback: (value: T) => Promise<(() => Promise<void>) | void>,
   ): void {
     let latestValue: T | typeof nothing = nothing;
-    let reconcilePromise: Promise<void> | undefined = undefined;
+    let running = false;
     let cleanUp: (() => Promise<void>) | void = undefined;
     let prevVal: T | typeof nothing = nothing;
 
     // While this loop runs it will process the latest from `value$` until it caught up with the updates.
     // It might skip updates from `value$` and only process the newest value after callback has resolved.
     const reconcileLoop = async (): Promise<void> => {
+      running = true;
+      try {
         while (latestValue !== prevVal) {
           await cleanUp?.(); // Call the previous value's clean-up handler
           prevVal = latestValue;
@@ -179,6 +181,12 @@ export class ObservableScope {
           if (latestValue !== nothing) cleanUp = await callback(latestValue); // Sync current value...
           // `latestValue` might have gotten updated during the `await callback`. That is why we loop here
         }
+      } finally {
+        // Cleared the moment the loop is done, not when its promise settles:
+        // a value arriving in between would otherwise find the loop still
+        // marked as running and never be processed
+        running = false;
+      }
     };
 
     value$
@@ -192,11 +200,8 @@ export class ObservableScope {
         latestValue = value;
         // There's already an instance of the below 'reconcileLoop' loop running
         // concurrently. So lets let the loop handle it. NEVER instanciate two `reconcileLoop`s.
-        if (reconcilePromise) return;
-
-        reconcilePromise = reconcileLoop().finally(() => {
-          reconcilePromise = undefined;
-        });
+        if (running) return;
+        void reconcileLoop();
       });
   }