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Use Selenium’s window-management API: call driver.set_window_size(width, height) before navigating, then verify with driver.get_window_size(). When you also need the window’s screen position, use set_window_rect() and get_window_rect(). The values describe the browser’s outer window, so operating-system window managers, headless mode and remote grids can affect the result.
Set a Selenium window size in Python
Install Selenium and create the driver, set the requested width and height in pixels, then navigate to the page whose responsive behavior you want to test.
from selenium import webdriver
# Selenium Manager can obtain a compatible Chrome driver in current Selenium releases.
driver = webdriver.Chrome()
try:
driver.set_window_size(1024, 768)
driver.get("https://example.com")
finally:
driver.quit()set_window_size(width, height) changes the current window’s width and height. Pass integers; fractional CSS pixels are not accepted by this API. Set the size before navigation when you are testing breakpoints, because the page may choose a layout during its initial load.
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Set only the dimensions
driver.set_window_size(1440, 900)This is the clearest choice when your test contract is simply “run at 1,440 × 900.” It does not intentionally move the window.
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Set position and dimensions together
driver.set_window_rect(x=40, y=20, width=1024, height=768)The rectangle API accepts any combination of x, y, width and height. For example, driver.set_window_rect(width=1000, height=700) changes only the dimensions. Rect commands are the W3C-style interface; older or non-W3C environments may require separate position and size calls.
Check the size Selenium actually applied
Read width and height
size = driver.get_window_size()
print(size) # {'width': ..., 'height': ...}
assert size["width"] == 1024
assert size["height"] == 768The returned dictionary represents the current window dimensions. Reading it back catches driver, operating-system and grid behavior that differs from the request.
Read the complete window rectangle
rect = driver.get_window_rect()
print(rect) # {'x': ..., 'y': ..., 'width': ..., 'height': ...}
assert rect["width"] == 1024
assert rect["height"] == 768Use a rectangle when a failure could be caused by placement, not just size. Logging x and y is useful when a desktop environment places windows on a different monitor or clamps them to the available work area.
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Exact equality is appropriate for a headed local test when outer-window dimensions are part of the contract. In CI, headless mode and remote execution, assert the requirement your test actually needs and log the returned rectangle:
rect = driver.get_window_rect()
print(f"window rect: {rect}")
# Example: require at least a desktop layout rather than one exact outer size.
assert rect["width"] >= 1000
assert rect["height"] >= 700If your application responds to the viewport rather than the outer window, verify the layout outcome (for example, which navigation element is visible) instead of assuming that an outer size maps to an identical viewport.
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A complete, reusable Python test pattern
from selenium import webdriver
from selenium.webdriver.common.by import By
WIDTH, HEIGHT = 1280, 800
driver = webdriver.Chrome()
try:
driver.set_window_size(WIDTH, HEIGHT)
driver.get("https://example.com")
rect = driver.get_window_rect()
print("Applied window rectangle:", rect)
# Exact checks are suitable only when this environment honors outer size.
if rect["width"] != WIDTH or rect["height"] != HEIGHT:
raise AssertionError(
f"Requested {WIDTH}x{HEIGHT}, got "
f"{rect['width']}x{rect['height']}"
)
# Continue with page assertions here.
print(driver.title)
finally:
driver.quit()Keep the resize before get(). If a test changes size after loading, wait for the responsive state to settle and then assert page behavior; a resize can trigger media-query changes, reflow and lazy-loading decisions.
Outer window size versus viewport size
Selenium’s window commands target the browser window. Browser chrome, borders, toolbars, display scaling and the operating system’s window manager consume part of that outer rectangle. Consequently, a requested 1024 × 768 window does not guarantee a 1024 × 768 CSS viewport.
When viewport dimensions are the real requirement
Responsive tests should define the contract explicitly:
- Use window commands when the requirement concerns the desktop window managed by the driver.
- Use a browser’s viewport emulation or a device preset when the requirement is a specific CSS viewport or mobile device profile.
- Assert visible breakpoints, element geometry or JavaScript-reported viewport values when those are what users experience.
Screen resolution can affect rendering, and the exact behavior of window commands is specific to the individual window manager. This is especially important on Linux CI hosts, containers, headless sessions and Selenium Grid nodes.
Headed, headless and remote execution
Headed local browsers
A visible browser usually gives the most intuitive relationship between requested outer dimensions and returned dimensions. The desktop can still constrain a window if the requested size exceeds the available work area.
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Headless browsers
Headless Chrome and other headless implementations may interpret window size as a virtual rendering surface. Configure the headless option supported by your browser when you need deterministic viewport behavior, then use Selenium’s read-back call to confirm what the driver reports. Do not infer a physical monitor size from a headless rectangle.
Remote grids and containers
The node, desktop session and driver version—not just your test code—control window management. Record the browser version, platform, requested values and returned rectangle in failure logs. If a grid cannot honor outer-window operations, move the assertion to viewport or page behavior and document that contract.
Equivalent commands in other Selenium bindings
The method names vary by language, but the same distinction applies: set width and height, then read them back; use a rectangle when position is also relevant.
| Binding | Set size | Read size |
|---|---|---|
| Java | driver.manage().window().setSize(new Dimension(1024, 768)); |
driver.manage().window().getSize() |
| Python | driver.set_window_size(1024, 768) |
driver.get_window_size() |
| C# | driver.Manage().Window.Size = new Size(1024, 768); |
driver.Manage().Window.Size |
| Ruby | driver.manage.window.resize_to(1024, 768) |
driver.manage.window.size |
| JavaScript | await driver.manage().window().setRect({width: 1024, height: 768}) |
await driver.manage().window().getRect() |
| Kotlin | driver.manage.window.size = Dimension(1024, 768) |
driver.manage.window.size |
JavaScript’s getRect() returns the rectangle object; a practical check is to assert that its width and height are positive before testing the page.
Troubleshooting common failures
The returned size differs from the requested size
Cause: the window manager, desktop work area, browser chrome, headless implementation or remote node constrained the outer window.
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Fix: call get_window_rect() immediately after resizing, log the result and test the actual contract. Reduce the requested size to fit the node, use a configured viewport for responsive tests, or run on a headed environment when outer-window equality is essential.
The command works locally but fails in CI
Cause: CI commonly runs headless or without a full desktop session, and grid nodes can have different display constraints.
Fix: capture browser, driver, operating-system and returned-rectangle details. Avoid hard-coding a physical-screen assumption; assert layout behavior or a minimum usable viewport instead.
The page uses the wrong responsive breakpoint
Cause: you resized after navigation, or the CSS viewport is smaller than the outer window because of browser chrome and scaling.
Fix: set size before get(), then verify the breakpoint-dependent element. If the test needs CSS-pixel precision, use viewport/device emulation rather than relying on outer dimensions alone.
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A rect command is rejected
Cause: an older driver or a non-W3C-compatible implementation may not support the complete rect command.
Fix: update Selenium and the browser driver where possible. Otherwise use the binding’s separate position and size methods, and keep a read-back check.
The browser cannot be created
Cause: Selenium cannot find or start a compatible browser/driver, or the CI user lacks a display.
Fix: verify the browser installation and driver compatibility, use Selenium Manager or a pinned driver strategy, and configure the browser’s headless mode for display-less hosts. This is a startup problem, not a window-size assertion problem.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing the right API
- Width and height only: use
set_window_sizeandget_window_size. - Position plus dimensions: use
set_window_rectandget_window_rect. - Responsive CSS behavior: set the size before navigation and assert the resulting UI; do not assume outer and viewport pixels are identical.
- Remote or headless reliability: log the returned rectangle and use environment-appropriate, contract-based assertions.
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Frequently Asked Questions
Should I call set_window_size before or after driver.get()?
Call it before navigation when the test depends on the initial responsive layout. Then read the applied rectangle and assert the page state you care about.
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Can Selenium set a mobile viewport with set_window_size alone?
No. It changes the browser window’s outer dimensions. Mobile viewport behavior generally requires a browser or device-emulation configuration in addition to window management.
What does get_window_rect add over get_window_size?
It returns x and y position along with width and height, which helps diagnose window-manager and remote-node constraints.
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