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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →If Selenium reports a 1024×768 Firefox window but the saved PNG is 1024×694, do not assume Selenium ignored your setting. A requested window size, headless Firefox’s virtual display, and the image’s capture scope are different measurements. Set and log the window explicitly, configure the headless display when needed, and inspect the output file itself.
What the different dimensions mean
Three dimensions are commonly confused:
- WebDriver window size: the width and height returned by
driver.get_window_size()after Selenium asks Firefox to resize. - Headless virtual display: the no-GUI display used by Firefox. Mozilla documents
MOZ_HEADLESS_WIDTHandMOZ_HEADLESS_HEIGHTfor this display. - Screenshot pixels and scope: the dimensions of the file written by
save_screenshot(), which depend on whether you capture the current window or the full document.
These values can differ without indicating a single universal Firefox bug. A 2020 geckodriver report recorded a 1024×768 reported window and a 1024×694 PNG on macOS 10.15.4 with Firefox 78.0.2, geckodriver 0.26.0, Selenium 3.141.0 and Python 3.7. That is one historical reproduction, not a rule that every headless session loses 74 pixels.
Reproduce and measure the mismatch
Use a minimal script and record every relevant version. The log lets you distinguish a resize problem from a screenshot-scope problem.
import platform
import sys
from pathlib import Path
from selenium import webdriver
from selenium.webdriver.firefox.options import Options
from PIL import Image
options = Options()
options.add_argument("--headless")
driver = webdriver.Firefox(options=options)
try:
print("Python:", sys.version)
print("OS:", platform.platform())
print("Firefox:", driver.capabilities.get("browserVersion"))
print("Geckodriver:", driver.capabilities.get("moz:geckodriverVersion"))
driver.set_window_size(1024, 768)
print("Reported size:", driver.get_window_size())
driver.get("https://example.com")
output = Path("window.png")
driver.save_screenshot(str(output))
print("PNG pixels:", Image.open(output).size)
finally:
driver.quit()Install Pillow only for the pixel check (python -m pip install selenium pillow). The browser and driver must already be installed and mutually compatible. The important comparison is the logged size versus the actual PNG dimensions; never infer file dimensions from the WebDriver response.
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Set the Selenium window size explicitly
- Create Firefox options and enable headless mode.
- Start the driver.
- Call
driver.set_window_size(width, height)before navigation or capture. - Immediately call
driver.get_window_size()and log the returned value. - Capture the file and inspect its pixel dimensions independently.
Selenium documents setting and retrieving window dimensions. If the returned value is already wrong, investigate driver or environment behavior before examining page layout. If it is correct but the PNG differs, continue with the display and scope checks below.
Configure Firefox’s headless display
Mozilla documents MOZ_HEADLESS_WIDTH and MOZ_HEADLESS_HEIGHT as controls for the dimensions of Firefox’s no-op virtual display. Set them in the process environment before creating the driver:
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import os
os.environ["MOZ_HEADLESS_WIDTH"] = "1024"
os.environ["MOZ_HEADLESS_HEIGHT"] = "768"
from selenium import webdriver
from selenium.webdriver.firefox.options import Options
options = Options()
options.add_argument("--headless")
driver = webdriver.Firefox(options=options)
driver.set_window_size(1024, 768)
print(driver.get_window_size())
# navigate and capture here
driver.quit()Keep these variables conceptually separate from Firefox’s command-line screenshot option. Mozilla’s command-line reference documents --window-size width[,height] for use with Firefox’s --screenshot command. That documentation does not establish that every command-line flag is interchangeable with Selenium’s WebDriver sizing API, so use the API and environment variables appropriate to your Selenium setup.
Choose the correct screenshot scope
Current-window capture
The general WebDriver screenshot operation captures the current browser window. Its height can represent the viewport or window area rather than the complete document. Browser chrome is not part of a headless PNG, and layout, device scale and driver implementation can affect the resulting pixel dimensions.
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driver.save_screenshot("viewport.png")Full-document capture
If the requirement is a page-length image, use the Firefox binding’s full-document screenshot method rather than treating a window screenshot as a full-page capture. Selenium’s Python Firefox API provides full-document screenshot methods; check the method available in your installed Selenium version.
# API names vary by Selenium version; inspect the Firefox WebDriver API
# for the full-document method supported by your binding.
driver.get_full_page_screenshot_as_file("page.png")A full-document image is expected to be taller than the viewport when the page scrolls. Therefore, a height mismatch may be correct behavior when the requested output changed from “visible window” to “entire document.”
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A diagnostic workflow that isolates the cause
- Capture environment details. Record operating system, Firefox, geckodriver, Selenium binding and language versions. Headless behavior is version-sensitive; the historical 1024×694 report cannot be generalized to current releases.
- Control the requested size. Set the dimensions with
set_window_size(), then printget_window_size(). - Control the virtual display. For headless sessions that need it, set
MOZ_HEADLESS_WIDTHandMOZ_HEADLESS_HEIGHTbefore driver startup. - Identify the target scope. Decide whether the deliverable is a current-window image or a full-document image.
- Measure the file. Read the PNG, JPEG or WebP dimensions with an image library or an image inspector.
- Preserve a minimal reproduction. Keep the URL, script, logs and exact versions if the difference remains. This is essential when reporting a geckodriver issue.
Common symptoms and fixes
| Symptom | Likely distinction | Action |
|---|---|---|
get_window_size() is not the requested value |
Resize was not applied or the driver adjusted it | Set the size after startup, log the returned value, and verify Firefox/geckodriver compatibility. |
| Window reports 1024×768; PNG is shorter | Current-window capture, headless display behavior, or a historical driver issue | Set the headless display variables, inspect the file pixels, and test with a minimal reproduction. |
| Image is much taller than the window | Full-document capture | Use current-window capture when a viewport image is required. |
| Image width is correct but page content is clipped | Viewport is narrower than the page’s responsive breakpoint | Set the intended width before navigation and verify the page’s CSS layout at that width. |
| Different machines produce different heights | OS, browser, driver or Selenium version differences | Log all versions and compare the same minimal script across environments. |
Reliability and performance considerations
- Set dimensions before loading the target page so responsive CSS and lazy layout calculations use the intended viewport.
- Wait for navigation and page content before capture; otherwise a short image can simply be an early document state.
- Use a deterministic test URL while diagnosing. Dynamic ads, consent dialogs and late-loading fonts can change layout height without changing the browser window.
- Do not “fix” a 1024×694 output by adding 74 pixels blindly. The historical difference is evidence about one environment, not a conversion constant.
- Compare lossless PNG dimensions first. JPEG and WebP may be appropriate for delivery, but compression settings do not explain a geometry mismatch.
- Full-document captures can require substantially more memory than viewport captures on long pages. Limit test pages and capture scope when running many jobs.
Troubleshooting checklist
- Is
--headlessenabled only once, through the Firefox options object? - Are
MOZ_HEADLESS_WIDTHandMOZ_HEADLESS_HEIGHTset beforewebdriver.Firefox()? - Did you log the value returned after
set_window_size()rather than only the requested arguments? - Did you inspect the actual image file rather than its filename or HTTP metadata?
- Are you calling a current-window method when you need full-document output, or vice versa?
- Can you reproduce the result with a fixed URL and the exact Firefox, geckodriver, Selenium and OS versions?
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Python, cURL and Node.js alternatives
Python
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
r.raise_for_status()
open("shot.webp", "wb").write(r.content)Node.js
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`${res.status} ${res.statusText}`);
const fs = await import('node:fs/promises');
await fs.writeFile('shot.webp', Buffer.from(await res.arrayBuffer()));Frequently asked questions
Does headless Firefox always remove part of the requested height?
No. The 1024×768 versus 1024×694 result comes from one 2020 report. Treat it as a reproduction to investigate, not a universal headless rule.
Which setting controls a Selenium screenshot: --window-size or the environment variables?
They document different interfaces: Selenium’s window API controls the WebDriver window, while Mozilla’s MOZ_HEADLESS_WIDTH and MOZ_HEADLESS_HEIGHT control the headless virtual display. Firefox’s --window-size documentation is specifically associated with the command-line --screenshot command.
How can I prove whether a mismatch is caused by the page or the browser?
Capture a fixed, simple page, log the returned window size, and read the image’s pixel dimensions. Then repeat with the same versions and settings on the other machine. This separates page layout changes from browser or driver behavior.
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