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Test Info:

<meta charset="utf-8">
<title>DPI of background page</title>
<script src="/tests/SimpleTest/SimpleTest.js"></script>
<script src="/tests/SimpleTest/ExtensionTestUtils.js"></script>
<script src="head.js"></script>
<link rel="stylesheet" href="/tests/SimpleTest/test.css">
"use strict";
async function testDPIMatches(description) {
let extension = ExtensionTestUtils.loadExtension({
background: function() {
browser.test.sendMessage("dpi", window.devicePixelRatio);
await extension.startup();
let dpi = await extension.awaitMessage("dpi");
await extension.unload();
// This assumes that the window is loaded in a device DPI.
`DPI in a background page should match DPI in primary chrome page ${description}`
add_task(async function test_dpi_simple() {
await testDPIMatches("by default");
add_task(async function test_dpi_devPixelsPerPx() {
await SpecialPowers.pushPrefEnv({
set: [["layout.css.devPixelsPerPx", 1.5]],
await testDPIMatches("with devPixelsPerPx");
await SpecialPowers.popPrefEnv();
add_task(async function test_dpi_os_zoom() {
await SpecialPowers.pushPrefEnv({ set: [["ui.textScaleFactor", 200]] });
await testDPIMatches("with OS zoom");
await SpecialPowers.popPrefEnv();