895277 - @media (min-resolution: 1.4dppx) doesn't apply when layout.css.devPixelsPerPx is set to 1.4

Status: NEW

(Core :: Layout, defect)

Description

[Jonathan Wilde [:jwilde]]

Attachment: failing testcase

Steps to reproduce:
- Open attached testcase (tested on Windows 8 only)
- Set layout.css.devPixelsPerPx to 1.4

Expected result:
Testcase turns green and says ">= 1.4 dppx", indicating that the @media query applied correctly.

Actual result:
Testcase stays blue and says "< 1.4 dppx". It doesn't start to turn green until layout.css.devPixelsPerPx is about 1.412.

Comment 1

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

I think the reality is that the devPixelsPerPx pref is rounded such that its reciprocal is the nearest number that is a multiple of 1/60.  (See nsDeviceContext::UpdateScaledAppUnits, which computes nsDeviceContext::mAppUnitsPerDevPixel.)

Comment 2

[Matt Brubeck (:mbrubeck)]

Windows 8 scales apps on high-dpi displays to 1.4dppx or 1.8dppx.  This rounding means that it is tricky for us to scale chrome or content in Firefox to match that resolution exactly.