1857656 - Consider keeping the line scroll amount constant in screen pixels when zoomed in

Status: NEW

(Core :: Layout: Scrolling and Overflow, enhancement, P3)

Description

[Botond Ballo [:botond]]

Steps to reproduce

1. Load https://theres-waldo.github.io/mozilla-testcases/long.html
2. At the default reflowing-zoom level (100%), tick the mousewheel once.
3. Take note of how many lines of text were scrolled.
4. Increase the reflowing-zoom level to 200%, and tick the mousewheel again.
5. Take note of how many lines of text were scrolled now.

The current behaviour is to keep the number of lines of text scrolled per mousewheel tick constant across different zoom levels, which results in the number of screen pixels scrolled per mousewheel tick not being constant across different zoom levels.

An alternative to consider would be to keep the number of screen pixels scrolled per mousewheel tick constant across different zoom levels. This is what Chrome appears to do.

A similar issue exists for pinch-zooming, though that's a different codepath and would require a different fix (if we want the same policy for it).