Platforms may report scroll deltas either in
chunks/lines/rows or pixels, depending on the
platform API and device capabilities.

If the platform reports a line/chunk-based delta
then the application needs to convert the delta
into a suitable number of pixels.

This commit just hardcodes it to 57 as
a starting point which matches the value that
Firefox calculates as the max char height
for the root frame on my system.

Fixes #5660