Capture full size screenshot is broken: Incorrect pixel dimensions

[guest271314]

Steps to reproduce:

1. Navigate to https://developer.mozilla.org/en-US/ 

2. CTRL+SHIFT+I (open DevTools).

3. Take note of document.body.clientHeight (2718) an document.body.clientWidth (1009).  Both initial values are incorrect, which we shall prove in the next steps.

4. CTRL+SHIFT+P (run command)

5. Type "Capture full size screenshot" then press ENTER

6. Take note of the resulting image pixel dimensions (1009 x 2718)

The resulting screenshot pixel dimensions are incorrect. That fact is observable at the clipped bottom of the image.

7. Open Mozilla Firefox or Nightly, repeat 1.

8. CTRL+SHIFT+K (open Web Console https://developer.mozilla.org/en-US/docs/Tools/Web_Console/Keyboard_shortcuts) 

9. Type ":screenshot --fullpage" then press ENTER

10. Take note of the resulting image pixel dimensions (1015 x 2752)

References: 

- capture screenshot of DOM #145 https://github.com/w3c/mediacapture-screen-share/issues/145#issuecomment-703361729

- Puppeteer takes partial page screenshot instead of full page #1273 https://github.com/puppeteer/puppeteer/issues/1273#issuecomment-703360535 et al.

See also

- Getting wrong window height in chrome https://stackoverflow.com/q/65453475

- CSS max-width has inaccurate pixel precision https://stackoverflow.com/q/65470317

Chromium 89.0.4384.0 (Developer Build) (64-bit) #841827

Firefox 84

[guest271314]

Chrome bug https://bugs.chromium.org/p/chromium/issues/detail?id=1164757