testlib: Workaound mysterious disappearing buttons

test/common/testlib.py

@@ -1357,9 +1357,31 @@ def relative_clips(sels: Collection[str]) -> Collection[tuple[int, int, int, int
         ref_filename = os.path.join(reference_dir, filename)
         self.used_pixel_references.add(ref_filename)
         rect["type"] = "box"
+        # HACK - https://github.com/cockpit-project/cockpit/issues/21577
+        #
+        # There is some really evil Chromium bug that often hides the
+        # primary button in dialogsscreenshots.  But funnily, calling
+        # captureScreenshot a second time will give us the correct
+        # rendering, every time.
+        #
+        # This doesn't seem to be a race between the page changing and
+        # us taking screenshots. No amount of waiting here helps
+        # fully. The second call to captureScreenshot seems to indeed
+        # trigger something that renders the page again, and correctly
+        # this time.
+        #
+        # If the PNG changes from the first call to the second, we
+        # warn about that. But note that not all these inconsistencies
+        # would lead to a pixel test failure.
+        #
+        ret1 = self.bidi("browsingContext.captureScreenshot", quiet=True,
+                         context=self.driver.top_context,
+                         clip=rect)
         ret = self.bidi("browsingContext.captureScreenshot", quiet=True,
                         context=self.driver.top_context,
                         clip=rect)
+        if ret1["data"] != ret["data"]:
+            print("WARNING: Inconsistent screenshots for", base)
         png_now = base64.standard_b64decode(ret["data"])
         png_ref = os.path.exists(ref_filename) and open(ref_filename, "rb").read()
         if not png_ref: