Fix the causes of intermittent test failures

tests/Motus.Tests/Browser/FirefoxIntegrationTests.cs

@@ -209,7 +209,20 @@ await page.SetContentAsync(
         Assert.IsTrue(await page.EvaluateAsync<bool>(Run("b")), "The script did not scroll to an element below the fold.");
         Assert.IsTrue(await page.EvaluateAsync<bool>(InView), "The element is still outside the viewport.");
 
-        var scrollY = await page.EvaluateAsync<double>("window.scrollY");
+        // Centering lands on a half pixel whenever the viewport's height is odd, and Firefox can
+        // round a half-pixel scroll position to a whole one after the scroll has finished, so a
+        // position read straight after centering can move by a pixel with nothing scrolling it.
+        // The element is put at a whole-pixel position off center instead: still in view, so the
+        // script must leave it alone, and far enough from center that a script that scrolled
+        // anyway would visibly move it, which re-centering an already centered element would not.
+        var scrollY = await page.EvaluateAsync<double>("""
+            (() => {
+                const top = document.getElementById('b').getBoundingClientRect().top + window.scrollY;
+                window.scrollTo({ top: Math.round(top) - 100, behavior: 'instant' });
+                return window.scrollY;
+            })()
+            """);
+        Assert.IsTrue(await page.EvaluateAsync<bool>(InView), "The element left the viewport when it was moved off center.");
         Assert.IsFalse(await page.EvaluateAsync<bool>(Run("b")), "The script scrolled an element that was already in view.");
         Assert.AreEqual(scrollY, await page.EvaluateAsync<double>("window.scrollY"));
 

tests/Motus.Tests/Context/TracingTests.cs

@@ -296,16 +296,27 @@ public async Task StartAsync_ConcurrentOnSameBrowser_SerializesViaGate()
         await Task.Delay(50);
         Assert.IsFalse(secondStartTask.IsCompleted, "Second StartAsync should be gated until the first stops.");
 
-        // Now stop the first and let the second proceed.
+        // Now stop the first and let the second proceed. The second start's answer is queued
+        // here, before the stop, because the stop releases the gate on its way out and the
+        // second start sends Tracing.start from that moment on another thread. Queued after
+        // awaiting the stop, the answer could arrive after the command it was meant for, which
+        // then waits out the transport's command timeout. The gate is what keeps the two
+        // answers in order: nothing can send Tracing.start until Tracing.end has been answered.
         _socket.QueueResponse("""{"id":2,"result":{}}"""); // first end
+        _socket.QueueResponse("""{"id":3,"result":{}}"""); // second start, once the gate opens
         var firstStopTask = first.StopAsync();
         await Task.Delay(50);
         _socket.Enqueue("""{"method":"Tracing.tracingComplete","params":{"dataLossOccurred":false}}""");
         await firstStopTask;
 
-        _socket.QueueResponse("""{"id":3,"result":{}}"""); // second start (now unblocked)
         await secondStartTask;
 
+        var sent = _socket.SentMessages.Select(bytes => System.Text.Encoding.UTF8.GetString(bytes)).ToList();
+        var firstEnd = sent.FindIndex(json => json.Contains("\"Tracing.end\""));
+        var secondStart = sent.FindLastIndex(json => json.Contains("\"Tracing.start\""));
+        Assert.IsTrue(firstEnd >= 0 && secondStart > firstEnd,
+            "The second Tracing.start went out before the first trace had ended.");
+
         // Cleanly stop the second so the gate is released for any teardown.
         _socket.QueueResponse("""{"id":4,"result":{}}""");
         var secondStopTask = second.StopAsync();