1395816 - Scrolling speed of scrollable elements different based on font-size

Status: UNCONFIRMED

(Core :: Layout, defect, P3)

Description

[Jon]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:55.0) Gecko/20100101 Firefox/55.0
Build ID: 20170824053838

Steps to reproduce:

Create a scrollable element (e.g. overflow:auto on a div with a fixed height). Set the font size of it or any parent element to 0. Attempt to scroll.

Now change the previously set font size to something else (e.g. 1000px). Scroll again.


Actual results:

The font-size affects the scrolling speed (smaller = slower, larger = faster).

Demo (tl;dr version)
https://codepen.io/jonmilner/pen/brzEpj

Demo (showing slow scroll speed of div with font-size set to 0)
https://codepen.io/jonmilner/pen/XaOmwL

Demo (showing fast scroll speed of div with font-size set to 1000px)
https://codepen.io/jonmilner/pen/brzEdK


Expected results:

The scrolling speed of a scrollable element should not be affected by the font-size of it or its parent, as this behavior is inconsistent with how scrolling works on the body (it is not affected by font-size).

Demo (showing normal scroll speed of body with font-size set to 0)
https://codepen.io/jonmilner/pen/BdMoXY

Demo (showing normal scroll speed of body with font-size set to 1000px)
https://codepen.io/jonmilner/pen/eExJpq