Preserve observed decoder lead while its pipeline drains

tools/chatgui-android/app/src/main/java/com/utun/chat/data/StreamDecodePump.kt

@@ -70,7 +70,8 @@ internal class StreamDecodePump<F : StreamDecodePump.Frame>(
                 output = next
                 if (!next.empty) {
                     if (firstOutputPts == null) firstOutputPts = next.pts
-                    outputCount++; decoderLeadUs = (lastInputPts - next.pts).coerceAtLeast(0)
+                    // Опустевший хвост не уменьшает измеренный защитный запас текущего decoder.
+                    outputCount++; decoderLeadUs = maxOf(decoderLeadUs, lastInputPts - next.pts)
                     while (submitted.isNotEmpty() && submitted.first <= next.pts) submitted.removeFirst()
                 }
                 waitingSince = now

tools/chatgui-android/app/src/test/java/com/utun/chat/data/StreamDecodePumpTest.kt

@@ -87,4 +87,12 @@ class StreamDecodePumpTest {
         codec.pump.step(40000, 160000); assertEquals(2, codec.released)
         codec.pump.step(80000, 200000); assertEquals(3, codec.released)
     }
+
+    @Test fun drainingDoesNotEraseTheObservedDecoderGuard() {
+        val codec = Codec(4)
+        repeat(4) { i -> codec.pump.offer(Frame(i * 40000L), i * 40000L); codec.pump.step(0, i * 40000L) }
+        assertEquals(120000L, codec.pump.decoderLeadUs); codec.finishing = true
+        codec.pump.step(120000, 200000)
+        assertEquals(4L, codec.pump.outputCount); assertEquals(120000L, codec.pump.decoderLeadUs)
+    }
 }