1648298 - Don't round subpixel sizes up in CSSOM APIs.

Status: NEW

(Core :: DOM: CSS Object Model, task, P3)

Description

[Emilio Cobos Álvarez (:emilio)]

Bug 1648265, caused by bug 1647356, exposed some interesting stuff.

At some places where we need to convert from CSS pixels to integers, like window.innerWidth / window.innerHeight, we're rounding, instead of truncating.

If websites use them to drive sizing decisions, like in that bug, this can cause undesired overflow at some zoom levels / resolutions.

I think ideally there should be two improvements:

• These sizes should be precisely available as floats to script.
• We shouldn't round up sizes like that when returning them to content.

It seems WebKit and Chromium (inheriting from WebKit, probably) truncates, unless you're really close to the next pixel (probably to account for floating point errors).

That seems somewhat hackish, but maybe reasonable?

Comment 1

[Emilio Cobos Álvarez (:emilio)]

Attachment: Bug 1648298 - WIP - Truncate instead of rounding for a bunch of CSS sizes that get converted to int.