test(nix): a failed ring check of wallpaper prints what it saw

The frame of the window, the hue that was found and the colors of the
pixels near its left edge, so that a port whose border is not repainted
(or is drawn elsewhere) can be told from one that has no border.

nix/tests/vm/lib/komorebi_vm/core/scenarios/wallpaper.py

@@ -88,13 +88,25 @@ def run(platform: Platform, checks: Checks) -> None:
         return platform.wait_for(lambda: hue_of(platform.screenshot(name)) == hue, timeout=20)
 
     def has_ring(client: Client, hue: str, name: str) -> bool:
-        """The window has a ring of `hue` in a screenshot taken until it does."""
+        """The window has a ring of `hue` in a screenshot taken until it does. When it has not,
+        what the last screenshot had is printed: the frame, and the colors it was read from."""
 
         def ring() -> bool:
             window = platform.os_window(client)
             return window is not None and ring_hue(window.frame, platform.screenshot(name)) == hue
 
-        return platform.wait_for(ring, timeout=20)
+        if platform.wait_for(ring, timeout=20):
+            return True
+        window = platform.os_window(client)
+        if window is None:
+            print(f"     {client.title} has no window")
+        else:
+            image = platform.screenshot(name)
+            f = window.frame
+            y = f.y + f.h // 2
+            row = [pixels.pixel(image, x, y) for x in range(max(0, f.x - 12), f.x + 12, 2)]
+            print(f"     {client.title} {f}: ring {ring_hue(f, image)}, wanted {hue}; pixels {row}")
+        return False
 
     platform.set_wallpaper(other)
     checks.check(