fix(web): re-point the widget diagnostic at the REAL document — the transparency premise gets verified

The one-shot dump + alpha log was bound to the FIRST capture ever = the initial
about:blank placeholder (alpha=0, rgb=0, 5/5 sessions) — a blind instrument. The
pre-parse transparency injection (1a39b09) is intact but was NEVER verified
(MyMistakes '? VERIFY'), and the 2026-09-10 take still shows a black, opaque
block. Stop guessing: NavigationCompleted for a non-about:blank document now
arms WidgetDumpRemaining=5; the next 5 post-paint captures dump
%TEMP%\ytLive-web-<id>-w1..5.png + shared AlphaStats(min/max/mean/zero%) +
FindContentBounds to startup.log.

The stats line alone names the branch: zero%≈opaque ⇒ the injection did not hold
for this widget's CSS (fix: !important stylesheet / chroma-key); large zero% +
tight bounds ⇒ capture IS transparent and the black lives in the compositor
blend. No new test: WebView2 runtime is not instantiable in the suite and the
re-point is log-only; 8/8 WebView2 tests still pass.

Services/WebView2Manager.cs

@@ -46,6 +46,7 @@ public sealed class WebView2Manager : IDisposable
         public long Epoch;
         public WriteableBitmap? PreviewBitmap;
         public bool DebugPngWritten;
+        public int WidgetDumpRemaining;
         public int TelemetryCaptureCount;
 
         public byte[] RentOutBuffer(int size)
@@ -179,9 +180,19 @@ public sealed class WebView2Manager : IDisposable
                 _ = cws.ExecuteScriptAsync(TransparentBackgroundScript);
             };
 
-            cws.NavigationCompleted += (_, _) =>
+            cws.NavigationCompleted += (_, e) =>
             {
                 _ = cws.ExecuteScriptAsync(TransparentBackgroundScript);
+                if (e.IsSuccess && session.Control.CoreWebView2?.Source is { } src && src != "about:blank")
+                {
+                    // Arm a one-shot diagnostic dump of the REAL widget document
+                    // (a few post-paint captures so styles/backgrounds settle) —
+                    // the old first-capture dump only ever saw the about:blank
+                    // document, so whether this widget's capture actually HAS the
+                    // transparent margins the injection promises has never been
+                    // verified (MyMistakes: "? VERIFY").
+                    session.WidgetDumpRemaining = 5;
+                }
                 _ = CaptureFrame(source.Id);
             };
 
@@ -257,6 +268,29 @@ public sealed class WebView2Manager : IDisposable
         return (minX, minY, maxX - minX + 1, maxY - minY + 1);
     }
 
+    // alpha[min/max/mean/zero%] — the one number that decides whether a widget's
+    // capture really has transparent margins (0% = opaque, ~100% if the page
+    // painted transparent-only). Sampled every 16th pixel like the original log.
+    private static string AlphaStats(byte[] pixels, int pixW, int pixH, int stride)
+    {
+        int minA = 255, maxA = 0;
+        long sumA = 0, nA = 0, zeroA = 0;
+        for (var y = 0; y < pixH; y += 16)
+        {
+            var row = y * stride;
+            for (var x = 0; x < pixW; x += 16)
+            {
+                var a = pixels[row + x * 4 + 3];
+                if (a < minA) minA = a;
+                if (a > maxA) maxA = a;
+                sumA += a;
+                if (a == 0) zeroA++;
+                nA++;
+            }
+        }
+        return $"alpha[min={minA},max={maxA},mean={sumA / (double)nA:F1},zero={100.0 * zeroA / nA:F1}%]";
+    }
+
     private async Task CaptureFrame(string sourceId)
     {
         if (!_sessions.TryGetValue(sourceId, out var session) || session.Disposed) return;
@@ -313,27 +347,31 @@ public sealed class WebView2Manager : IDisposable
 
             if (session.Epoch == 0)
             {
-                int minA = 255, maxA = 0;
-                long sumA = 0, nA = 0, zeroA = 0;
-                for (var y = 0; y < pixH; y += 16)
-                {
-                    var row = y * stride;
-                    for (var x = 0; x < pixW; x += 16)
-                    {
-                        var a = pixels[row + x * 4 + 3];
-                        if (a < minA) minA = a;
-                        if (a > maxA) maxA = a;
-                        sumA += a;
-                        if (a == 0) zeroA++;
-                        nA++;
-                    }
-                }
                 AppLog.Write(
                     $"WebView2Manager: first capture for '{sourceId}' {pixW}x{pixH} " +
-                    $"alpha[min={minA},max={maxA},mean={sumA / (double)nA:F1},zero={100.0 * zeroA / nA:F1}%] " +
+                    $"{AlphaStats(pixels, pixW, pixH, stride)} " +
                     $"contentBounds=({cropX},{cropY},{cropW},{cropH})");
             }
 
+            if (session.WidgetDumpRemaining > 0)
+            {
+                var dumpNo = 5 - session.WidgetDumpRemaining + 1;
+                session.WidgetDumpRemaining--;
+                try
+                {
+                    var dbg = Path.Combine(Path.GetTempPath(), $"ytLive-web-{sourceId}-w{dumpNo}.png");
+                    File.WriteAllBytes(dbg, ms.ToArray());
+                    AppLog.Write(
+                        $"WebView2Manager: widget capture [{dumpNo}/5] for '{sourceId}' {pixW}x{pixH} " +
+                        $"{AlphaStats(pixels, pixW, pixH, stride)} " +
+                        $"contentBounds=({cropX},{cropY},{cropW},{cropH}) -> {dbg}");
+                }
+                catch (Exception dex)
+                {
+                    AppLog.Write($"WebView2Manager: widget dump failed: {dex.Message}");
+                }
+            }
+
             var outPixels = session.RentOutBuffer(cropW * cropH * 4);
             if (cropW == pixW && cropH == pixH && cropX == 0 && cropY == 0)
                 Buffer.BlockCopy(pixels, 0, outPixels, 0, outPixels.Length);