1657380 - CSS screen.height seems wrong

Status: NEW

(Core :: Layout, defect, P2)

Description

[James Willcox (:snorp) ([e-mail]) (he/him)]

According to the discussion here[1], we seem to be subtracting 16 pixels from the value returned from screen.height compared to other browsers.

[1] https://github.com/w3c/csswg-drafts/issues/5204

Comment 1

[Makoto Kato [:m_kato]]

This seems to be rounding issue of int vs float/double.

nsDeviceContext::SetDPI calculates mAppUnitsPerDevPixelAtUnitFullZoom, it is int value, it causes rounding issue on some device.
When using Pixel 2, dpi density is 2.625. AppUnitsPerCSSPixel is 60, so 60 % 2.625 != 0.

When device pixel is 1920x1080 and dpi is 2.625,

Gecko: 1920 * mAppUnitsPerDevPixelAtUnitFullZoom (23) / 60 = 736px
Blink: 1920 / 2.625 = 731.428571429.. = 732px

Comment 2

[Makoto Kato [:m_kato]]

What's owner of AppUnits rouding issue? gfx?, layout?

Also, I guess that window.devicePixelRatio doesn't equals to Blink's on Pixel 4a etc due to same rounding issue of nsPresContext.