screenshot: add unit test for FullScreenshot

This unit test is copied from puppeteer:
- testdata/grid.html
- testdata/digits/[0-9].png
- testdata/screenshots/grid-fullpage.png

screenshot_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"image"
+	_ "image/jpeg"
 	"image/png"
 	"os"
 	"path"
@@ -114,6 +115,53 @@ func TestCaptureScreenshot(t *testing.T) {
 	}
 }
 
+func TestFullScreenshot(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		quality int
+		want    string
+	}{
+		{
+			name:    "quality 100",
+			quality: 100,
+			want:    "grid-fullpage.png",
+		},
+		{
+			name:    "quality 90",
+			quality: 90,
+			want:    "grid-fullpage-90.jpeg",
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			ctx, cancel := testAllocate(t, "grid.html")
+			defer cancel()
+
+			var buf []byte
+			if err := Run(ctx,
+				EmulateViewport(500, 500),
+				EvaluateAsDevTools("document.documentElement.scrollTo(20,  30)", nil),
+				FullScreenshot(&buf, test.quality),
+			); err != nil {
+				t.Fatal(err)
+			}
+
+			diff, err := matchPixel(buf, test.want)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if diff != 0 {
+				t.Fatalf("screenshot does not match. diff: %v", diff)
+			}
+		})
+	}
+}
+
 func matchPixel(buf []byte, want string) (int, error) {
 	img1, format1, err := image.Decode(bytes.NewReader(buf))
 	if err != nil {

testdata/grid.html

@@ -0,0 +1,52 @@
+<script>
+document.addEventListener('DOMContentLoaded', function() {
+    function generatePalette(amount) {
+        var result = [];
+        var hueStep = 360 / amount;
+        for (var i = 0; i < amount; ++i)
+            result.push('hsl(' + (hueStep * i) + ', 100%, 90%)');
+        return result;
+    }
+
+    var palette = generatePalette(100);
+    for (var i = 0; i < 200; ++i) {
+        var box = document.createElement('div');
+        box.classList.add('box');
+        box.style.setProperty('background-color', palette[i % palette.length]);
+        var x = i;
+        do {
+            var digit = x % 10;
+            x = (x / 10)|0;
+            var img = document.createElement('img');
+            img.src = `./digits/${digit}.png`;
+            box.insertBefore(img, box.firstChild);
+        } while (x);
+        document.body.appendChild(box);
+    }
+});
+</script>
+
+<style>
+
+body {
+    margin: 0;
+    padding: 0;
+}
+
+.box {
+    font-family: arial;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    margin: 0;
+    padding: 0;
+    width: 50px;
+    height: 50px;
+    box-sizing: border-box;
+    border: 1px solid darkgray;
+}
+
+::-webkit-scrollbar {
+    display: none;
+}
+</style>