test(sidecar): pin that a slow read counts toward the stability streak

sidecar/src/main/kotlin/dev/sanderling/sidecar/DriverBackend.kt

@@ -88,8 +88,14 @@ internal const val TRANSITION_POLL_INTERVAL_MILLIS = 100L
 // NavHost cross-fade is a 700ms tween (Compose navigation's default enter and
 // exit), it starts when the action lands rather than when the snapshot begins,
 // and the streak above has to fit after it. Measured on the emulator, a 600ms
-// cap left about a third of fades unfinished. A layout that holds two routes at
+// cap left about a third of fades unfinished.
-// rest costs the full cap once per step and no more.
+//
+// A layout that holds two routes at rest pays the full cap on every snapshot
+// RPC it is read with, and the runner issues more than one: fetchSyncedState
+// (internal/runner) re-fetches a tree it considers transitional up to 4 times.
+// It stops early once two consecutive fetches come back byte-identical, so such
+// a layout costs 2 caps on a still tree and up to 4 on one that jitters
+// underneath. Per step, not once per step.
 internal const val TRANSITION_POLL_CAP_MILLIS = 1500L
 
 // awaitSettledTree reads the hierarchy and, while the tree it gets back holds
@@ -123,11 +129,14 @@ internal fun awaitSettledTree(read: () -> String): String {
 // state stable.
 //
 // The streak is measured from the start of the read that opened the current
-// run of identical snapshots, not from when that read returned: a hierarchy
+// run of identical snapshots, not from when that read returned, so a slow read
-// fetch is not instantaneous, and the UI changing mid-fetch would have changed
+// is charged to the streak. That is a deliberate trade and not a free one: the
-// the snapshot, so the fetch's own duration is evidence of stability. On
+// quiet the poll actually OBSERVED spans the last read's start back to the
-// Android a fetch costs more than the poll interval, so charging it to the
+// first read's return, which is shorter than streakMillis by up to the two
-// streak is the difference between two reads and four.
+// reads' durations. On Android a hierarchy fetch costs more than the poll
+// interval, so charging it is the difference between two reads and four, and a
+// caller wanting the full streak observed has to widen streakMillis rather than
+// assume it. StabilityPollTest.slowSnapshotReadsCountTowardTheStreak pins this.
 internal fun pollUntilStable(
     timeoutMillis: Long,
     streakMillis: Long = MIN_STABLE_STREAK_MILLIS,

sidecar/src/test/kotlin/dev/sanderling/sidecar/StabilityPollTest.kt

@@ -16,6 +16,46 @@ class StabilityPollTest {
         assertTrue(elapsed < 3000L, "should not run to cap when stable, elapsed=${elapsed}ms")
     }
 
+    @Test fun slowSnapshotReadsCountTowardTheStreak() {
+        // The streak runs from the START of the read that opened the run of
+        // identical snapshots, so the read's own duration is charged to it.
+        // Every other test here uses an instant lambda and so passes under
+        // either semantics; this one is the difference. StubDriverBackend's
+        // waitForIdle polls a real `uiautomator dump`, which costs hundreds of
+        // milliseconds, so the slow read is the case it runs in.
+        val readMillis = 400L
+        val sampleStarts = mutableListOf<Long>()
+        val sampleEnds = mutableListOf<Long>()
+        val start = System.currentTimeMillis()
+        pollUntilStable(5000L) {
+            sampleStarts += System.currentTimeMillis() - start
+            Thread.sleep(readMillis)
+            sampleEnds += System.currentTimeMillis() - start
+            "stable"
+        }
+        val elapsed = System.currentTimeMillis() - start
+
+        // One read plus one interval plus one read already clears 750ms, so the
+        // poll settles for two samples where an instant read takes four.
+        assertEquals(
+            2,
+            sampleStarts.size,
+            "a ${readMillis}ms read should clear the streak in two samples, starts=$sampleStarts",
+        )
+        assertTrue(elapsed >= MIN_STABLE_STREAK_MILLIS, "elapsed=${elapsed}ms")
+
+        // What the poll actually watched: the last read began this long after
+        // the first one returned. It is the poll interval, not the streak, and
+        // a caller that needs MIN_STABLE_STREAK_MILLIS of observed quiet has to
+        // ask for a wider streak rather than assume this one delivers it.
+        val observedQuiet = sampleStarts.last() - sampleEnds.first()
+        assertTrue(
+            observedQuiet < MIN_STABLE_STREAK_MILLIS,
+            "the poll returned having observed ${observedQuiet}ms of quiet, not " +
+                "${MIN_STABLE_STREAK_MILLIS}ms; if that changed, the streak semantics changed",
+        )
+    }
+
     @Test fun streakResetsOnAnyChange() {
         // A late transition that fires after the prior streak has already
         // begun must reset the clock: the post-transition stable window has