I have two 1920 x 1080 monitors, one horizontal and one vertical. When visiting this viewport measuring site on on my horizontal one it says 1920 × 947 which is normal. On my vertical one though it says
864 x 1403.
The viewport width is over 200 pixels smaller than it should be! I see no visual clue that could explain such a weird phenomenon: Chrome window correctly fills the screen, the web pages are complete, etc.
Below is a screenshot taken from another site that measures viewport width that shows both the viewport width (847px) calculated by that particular website, and the actual width of the screen as indicated by my screenshot app (1058px).
This looks like black magic. If anybody has the slightest clue please come forward!