Fix TestPageScreenshotFullpage on Windows

js/modules/k6/browser/tests/page_test.go

@@ -673,7 +673,7 @@ func TestPageScreenshotFullpage(t *testing.T) {
 		const div = document.createElement('div');
 		div.style.width = '1280px';
 		div.style.height = '800px';
-		div.style.background = 'linear-gradient(red, blue)';
+		div.style.background = 'linear-gradient(to bottom, red, blue)';
 
 		document.body.appendChild(div);
 	}`)
@@ -688,15 +688,16 @@ func TestPageScreenshotFullpage(t *testing.T) {
 	img, err := png.Decode(reader)
 	assert.Nil(t, err)
 
-	assert.Equal(t, 1280, img.Bounds().Max.X, "screenshot width is not 1280px as expected, but %dpx", img.Bounds().Max.X)
-	assert.Equal(t, 800, img.Bounds().Max.Y, "screenshot height is not 800px as expected, but %dpx", img.Bounds().Max.Y)
+	assert.Equal(t, 1280, img.Bounds().Max.X, "want: screenshot width is 1280px, got: %dpx", img.Bounds().Max.X)
+	assert.Equal(t, 800, img.Bounds().Max.Y, "want: screenshot height is 800px, got: %dpx", img.Bounds().Max.Y)
 
+	// Allow tolerance to account for differences in rendering between
+	// different platforms and browsers. The goal is to ensure that the
+	// screenshot is mostly red at the top and mostly blue at the bottom.
 	r, _, b, _ := img.At(0, 0).RGBA()
-	assert.Greater(t, r, uint32(128))
-	assert.Less(t, b, uint32(128))
+	require.Truef(t, r > b*2, "want: the top pixel to be dominantly red, got R: %d, B: %d", r, b)
 	r, _, b, _ = img.At(0, 799).RGBA()
-	assert.Less(t, r, uint32(128))
-	assert.Greater(t, b, uint32(128))
+	require.Truef(t, b > r*2, "want: the bottom pixel to be dominantly blue, got R: %d, B: %d", r, b)
 }
 
 func TestPageTitle(t *testing.T) {