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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsIf a Selenium PNG is twice as wide and twice as tall as the viewport you configured, first determine whether the result is an old ChromeDriver defect, intentional device emulation, or the expected high-density output of a Retina environment. Record your Chrome, ChromeDriver, Selenium, operating-system, and headed/headless versions; read back the WebDriver window and page metrics; inspect window.devicePixelRatio; and compare those values with the actual PNG dimensions. ChromeDriver 78.0.3904.11 specifically listed “Fixed screenshot size on retina display,” but that historical release note is not proof of a current universal bug. Configure the output your tests actually need—CSS-pixel-sized or high-density—and assert the dimensions in the same environment used by CI.
What “double resolution” means
WebDriver exposes sizes in CSS pixels, while a screenshot file contains device pixels. On a Retina display, a page that is 1,440 CSS pixels wide can produce a 2,880-pixel-wide bitmap when the effective device pixel ratio is 2. If both width and height double, the bitmap contains four times as many pixels. That arithmetic explains one common symptom; it does not prove that every Retina capture should be downsampled.
- Window dimensions: the outer browser window size controlled through WebDriver.
- Viewport dimensions: the page’s CSS layout area, available through JavaScript.
- Device pixel ratio (DPR): the browser’s CSS-pixel-to-device-pixel scale, reported by
window.devicePixelRatio. - PNG dimensions: the physical pixel width and height stored in the file.
Decide which artifact your test requires. A CSS-pixel baseline is usually easier to compare across machines. A high-density baseline preserves Retina detail and is valid when your consumer expects device-pixel output. The fix is to make that choice explicit and reproduce it, not to apply a flag blindly.
Check versions before changing scale
Capture the versions and mode from the machine that produces the unexpected image. An old driver can create behavior that looks like display scaling. ChromeDriver’s official release history records the entry “Fixed screenshot size on retina display” for ChromeDriver 78.0.3904.11, a driver release supporting Chrome 78. Treat this as a historical, version-specific clue. Align a compatible driver and browser before diagnosing a modern setup.
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Use your package manager or Selenium’s capabilities to record:
- Chrome’s exact version and ChromeDriver’s exact version;
- Selenium language binding and version;
- operating system and whether the session is headed or headless;
- local versus CI execution, including the CI display or virtual display configuration.
ChromeDriver’s release history is the authoritative place to check older driver notes. Do not assume that a current browser has the same defect or that one launch argument fixes all platforms.
Measure the browser, page, and file
Read back the WebDriver rectangle
Set a known window size, then read the actual rectangle instead of trusting the requested value. Selenium documents window management and notes that screen resolution can affect rendering in its window documentation.
from selenium import webdriver
from selenium.webdriver.chrome.options import Options
options = Options()
# Keep this commented while diagnosing headed Retina behavior.
# options.add_argument("--headless=new")
driver = webdriver.Chrome(options=options)
driver.set_window_size(1440, 900)
print("window rectangle:", driver.get_window_rect())
print("inner size:", driver.execute_script(
"return {width: window.innerWidth, height: window.innerHeight, "
"dpr: window.devicePixelRatio, outerWidth: window.outerWidth, "
"outerHeight: window.outerHeight};"
))
driver.get("https://example.com")
driver.save_screenshot("diagnostic.png")
driver.quit()set_window_size, rectangle methods, and saving the current-window screenshot are documented in Selenium’s Python Chromium WebDriver API. The example intentionally prints both outer and inner dimensions: browser chrome consumes part of the outer rectangle, so neither value alone is the PNG’s guaranteed size.
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Inspect CSS dimensions and DPR in the page
Run this after navigation, because a page can change its viewport or enter fullscreen-like behavior after it loads:
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metrics = driver.execute_script("""
return {
innerWidth: window.innerWidth,
innerHeight: window.innerHeight,
clientWidth: document.documentElement.clientWidth,
clientHeight: document.documentElement.clientHeight,
devicePixelRatio: window.devicePixelRatio,
screenWidth: window.screen.width,
screenHeight: window.screen.height
};
""")
print(metrics)Now inspect the file itself. For example, with Pillow:
from PIL import Image
with Image.open("diagnostic.png") as image:
print("PNG pixels:", image.size)
print("PNG mode:", image.mode)Compare the PNG width and height with innerWidth/innerHeight and DPR. If the bitmap is approximately twice each CSS dimension while DPR is approximately 2, you have identified a high-density capture path. If the numbers disagree, investigate window decorations, full-page capture behavior, emulation, or a driver/browser mismatch rather than simply resizing the image.
Check Chrome mobile emulation and device metrics
ChromeDriver supports mobile emulation with individual device metrics, including a pixelRatio value described as the emulated device’s pixel ratio. See the ChromeDriver mobile-emulation documentation. Emulation is relevant only if your test enables it; ordinary desktop sessions should not turn it on just to compensate for a Retina monitor.
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from selenium import webdriver
from selenium.webdriver.chrome.options import Options
options = Options()
options.add_experimental_option("mobileEmulation", {
"deviceMetrics": {
"width": 390,
"height": 844,
"pixelRatio": 3
},
"userAgent": "Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) "
"AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 "
"Mobile/15E148 Safari/604.1"
})
driver = webdriver.Chrome(options=options)Remove the emulation block when the test is intended to represent desktop Chrome. When it is intentional, document the selected width, height, and pixelRatio as part of the baseline; changing any of them changes layout and image dimensions.
Choose and apply the right correction
When a historical driver/browser pair is involved
Upgrade or align ChromeDriver and Chrome according to the release history, then rerun the dimension check. Do not mix a newly installed browser with a stale driver binary and infer a scaling problem from the resulting error or screenshot.
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When you need CSS-pixel-sized baselines
- Use a fixed WebDriver window size and read back the rectangle.
- Run with the same headed/headless mode in development and CI.
- Disable unintended mobile emulation and verify DPR.
- Capture a screenshot and assert its actual dimensions rather than assuming they equal the requested window size.
A dimension assertion turns a silent Retina change into a visible test failure:
from PIL import Image
expected = (1440, 900) # Replace with the dimensions your capture contract requires.
driver.save_screenshot("page.png")
with Image.open("page.png") as image:
if image.size != expected:
raise AssertionError(f"expected {expected}, got {image.size}")Use the measured viewport, not an arbitrary number, when browser chrome or a headless mode makes the screenshot’s contract different from the outer window.
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Keep the high-density configuration and make it part of the specification: browser and driver versions, viewport, DPR, headed/headless mode, fonts, operating system, and CI display setup. Store baselines generated under that same contract. Downsampling can make files match a CSS-pixel comparison, but it discards device-pixel detail and can hide a real rendering change; it is a policy choice, not a universal repair.
Make captures reproducible in CI
- Pin the browser and driver: record exact versions and update them together.
- Pin geometry: set the window, verify the rectangle, and log viewport metrics.
- Pin rendering inputs: use the same fonts, operating system image, color scheme, timezone, and locale where visual diffs matter.
- Pin execution mode: headed and headless sessions can have different geometry and rendering paths.
- Pin emulation: either remove it or record every device metric, especially
pixelRatio. - Validate artifacts: assert width and height, and retain the metrics log beside failed images.
WebDriver sizing alone cannot guarantee identical pixels across platforms. A stable visual baseline is a complete environment contract, not just a call to set_window_size.
Troubleshooting common symptoms
The PNG is exactly twice the CSS width and height
Check DPR first. A DPR near 2 with a two-times bitmap is consistent with high-density output. Decide whether that output is desired, then standardize the environment or assert a high-density baseline.
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Changing the window size has no effect
Print get_window_rect() and the JavaScript metrics after navigation. A window manager, a headless mode, mobile emulation, or a page that changes viewport behavior may be overriding the requested geometry. Fix the first value that differs from your contract.
DPR is unexpected
Search Chrome options and capabilities for mobile emulation and device metrics. Remove accidental emulation, or set an explicit, documented pixelRatio for an emulated device. Do not infer desktop Retina behavior from a mobile profile.
Local captures pass but CI captures fail
Compare browser/driver versions, headed versus headless mode, virtual display resolution, fonts, and OS image. Log all metrics from both environments and make the CI image the source of truth for its baselines.
Updating ChromeDriver does not solve it
The 78.0.3904.11 note is historical. If versions are already aligned, inspect DPR, emulation, viewport, and PNG dimensions. The available documentation does not establish one universal current launch flag for every Chrome version and platform, so label any flag recipe with the exact environment in which it was verified.
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FAQ
Is a two-times PNG necessarily wrong?
No. It is expected when the capture contract calls for device-pixel detail and the effective DPR is about 2. It is wrong only when your test requires CSS-pixel dimensions.
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No universal flag combination is established for all current Chrome versions and platform modes. Verify the actual dimensions and DPR in the environment you ship.
Can one baseline serve every operating system?
Not reliably. Fonts, rasterization, browser mode, display configuration, and versions can alter pixels even with the same CSS viewport. Use a controlled rendering environment for strict image comparisons.
Frequently Asked Questions
Is a two-times PNG necessarily wrong?
No. It is expected when the capture contract calls for device-pixel detail and the effective DPR is about 2. It is wrong only when your test requires CSS-pixel dimensions.
Should I always add a device-scale-factor launch flag?
No universal flag combination is established for all current Chrome versions and platform modes. Verify the actual dimensions and DPR in the environment you ship.
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Not reliably. Fonts, rasterization, browser mode, display configuration, and versions can alter pixels even with the same CSS viewport. Use a controlled rendering environment for strict image comparisons.
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