Screen/canvas resolution not accurate

[Magicrafter13]

In portrait it reports 408x875, and in landscape 875x408.

For reference, my screen resolution is 1440p (21:9 ratio), and I'm getting screen information from here: http://www.lagom.nl/lcd-test/display_settings.php

[akliuxingyuan]

http://dev.jeffersonscher.com/resolution.html ref: https://support.mozilla.org/en-US/questions/981038