Because Selenium’s window size and a screenshot’s pixel dimensions are different measurements. Set the browser window, check the page’s actual viewport after navigation, and inspect the saved image. Also confirm whether you need a screenshot of the visible viewport or of the full document.
First, identify what is the wrong size
“Wrong size” can describe three different problems:
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- PNG dimensions: the file’s pixel width or height differs from what you expected.
- Visible content: the screenshot shows only part of the page, such as the top portion.
- Responsive layout: the site uses a mobile or tablet layout when you expected a desktop layout, or vice versa.
These symptoms have different causes. A screenshot can have the expected pixel dimensions but still show an unexpected responsive layout; a viewport screenshot can have the expected size while omitting content farther down the document.
Set the window, then measure the viewport
Selenium’s set_window_size(width, height) sets the current browser window’s dimensions. It is not a direct command to set the webpage’s rendered viewport to exactly those dimensions. Browser chrome, the operating system, or a virtual display can affect the space available to the page. Selenium also notes that screen resolution can affect how an application renders (Selenium window interactions).
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In Python, set the window and query the page’s viewport after navigation. The following example also records the actual saved PNG dimensions using Pillow (python -m pip install selenium pillow):
from pathlib import Path
from PIL import Image
from selenium import webdriver
options = webdriver.ChromeOptions()
# For a headed run, remove the next line.
options.add_argument("--headless=new")
driver = webdriver.Chrome(options=options)
try:
driver.set_window_size(1440, 1000)
driver.get("https://example.com")
viewport = driver.execute_script(
"return {width: window.innerWidth, height: window.innerHeight, "
"devicePixelRatio: window.devicePixelRatio}"
)
print("Requested window:", driver.get_window_size())
print("Rendered viewport:", viewport)
output = Path("page.png")
driver.save_screenshot(str(output))
with Image.open(output) as image:
print("Saved PNG pixels:", image.size)
finally:
driver.quit()The example uses Selenium’s Python binding. Selenium documents window-size examples in several language bindings; check the corresponding binding’s API for syntax (window interaction documentation). Python’s API describes set_window_size as setting the current window’s width and height, and its screenshot method saves the current browsing context (Selenium Python API, version 4.50.0).
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Interpret the measurements
- Viewport dimensions are unexpected: Check that the window-size command ran, inspect the size after navigation, and look for operating-system or virtual-display limits. A site’s responsive breakpoints respond to the page viewport, not simply to the dimensions you requested for the outer window.
- Viewport is right, PNG dimensions are unexpected: Investigate the browser and driver’s screenshot behavior and the runtime’s pixel scaling.
window.devicePixelRatiois useful diagnostic information, but the sources do not establish a universal Selenium rule tying it to screenshot dimensions; verify the result for your browser-driver combination. - PNG dimensions are right, layout is wrong: Compare the measured viewport with the width needed for the intended responsive breakpoint. Also check that the test is using the expected browser, headless mode, and display environment.
- Only the top of the page appears: You may have captured the visible viewport rather than the full document.
For repeatable comparisons, record the requested window size, measured viewport, saved image dimensions, browser and driver versions, operating system or container, headless setting, and screenshot method. Selenium’s documentation specifically cautions that screen resolution can affect rendering (Selenium window interactions).
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A regular screenshot captures the current browsing context. That is suitable when you want the page as it appears in the visible browser area. It does not mean that the image automatically includes every element below the fold.
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If you need the whole document, use a full-page method supported by your browser-specific Selenium API. Selenium’s Firefox API documents get_full_page_screenshot_as_file(...) separately (Firefox WebDriver API). Do not assume that this Firefox-specific method is identically available in every browser binding. Confirm support in the API for the browser and language you actually run.
Troubleshoot common mismatches
The browser window is smaller than requested
Log driver.get_window_size() after setting the size. If the returned dimensions are not what you requested, check the browser’s window state, display resolution, and any container or virtual-display configuration. Window resizing and fullscreen behavior are browser-environment dependent; compare results in the same runtime you use for screenshots.
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The page uses the wrong responsive layout
Read window.innerWidth after navigation and compare it with the layout width you intend to test. If the viewport is narrower than expected, investigate the window configuration and display limits rather than assuming the PNG file’s dimensions determine the CSS layout.
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The image is clipped vertically
Check whether your code calls the standard screenshot method or a browser-supported full-document method. A viewport screenshot and a full-page screenshot have different scopes; choose the one the test requires.
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Hold the environment steady and log the browser and driver versions, OS or container, headless mode, requested window size, measured viewport, and output image dimensions. Different screen resolutions can affect rendering, so compare like with like rather than relying on a window-size request alone.
Viewport and image pixel dimensions do not line up
Record window.devicePixelRatio and inspect the actual PNG dimensions. Pixel scaling is a diagnostic possibility, not a universal Selenium explanation; the behavior must be checked with the specific browser and driver in use.
Or skip the browser setup
If you need a screenshot of a public URL without configuring WebDriver, ScreenshotNeo is a website screenshot API. A single GET request can return an image or PDF. For example, save a WebP capture of a page:
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Frequently Asked Questions
Does Selenium’s window-size setting set the page viewport?
No. It sets the browser window dimensions. Measure the rendered page viewport separately after navigation.
Does a normal Selenium screenshot include the entire webpage?
Not necessarily. A regular screenshot captures the current browsing context; full-document capture requires a supported browser-specific method.
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