Reduce flaky screenshots. (dart-lang#8603)

Author: isoos

pkg/pub_integration/lib/src/screenshot_utils.dart

@@ -68,7 +68,10 @@ extension ScreenshotElementHandleExt on ElementHandle {
     final body = await page.$('body');
     final bodyClassAttr =
         (await body.evaluate('el => el.getAttribute("class")')) as String;
-    final bodyClasses = bodyClassAttr.split(' ');
+    final bodyClasses = [
+      ...bodyClassAttr.split(' '),
+      '-pub-ongoing-screenshot',
+    ];
 
     for (final vp in _viewports.entries) {
       await page.setViewport(vp.value);
@@ -84,10 +87,17 @@ extension ScreenshotElementHandleExt on ElementHandle {
         // The presence of the element is verified, continue only if screenshots are enabled.
         if (!_isScreenshotDirSet) continue;
 
+        // Arbitrary delay in the hope that potential ongoing updates complete.
+        await Future.delayed(Duration(milliseconds: 500));
+
         final path = p.join(_screenshotDir!, '$prefix-${vp.key}-$theme.png');
         await _writeScreenshotToFile(path);
       }
     }
+
+    // restore the original body class attributes
+    await body.evaluate('(el, v) => el.setAttribute("class", v)',
+        args: [bodyClassAttr]);
   }
 
   Future<void> _writeScreenshotToFile(String path) async {

pkg/web_css/lib/src/_base.scss

@@ -480,3 +480,13 @@ a.-x-ago {
     margin-left: 36px;
   }
 }
+
+.-pub-ongoing-screenshot {
+  // Disables flashing caret (cursor) in input fields by making it invisible.
+  caret-color: transparent;
+
+  // Instantaneous transitions - no ambiguous delay in the end state.
+  * {
+    transition-duration: 0s !important;
+  }
+}

pkg/web_css/test/expression_test.dart

@@ -57,6 +57,8 @@ void main() {
         'cookie-notice-container',
         'cookie-notice-button',
       ]);
+      // test-only style
+      expressions.remove('-pub-ongoing-screenshot');
       // markdown alert classes
       expressions.removeAll([
         'markdown-alert-note',