Using the horizontal scroll on a mouse with the HTML client ends up sending mouse buttons 4/5, not 6/7 as is expected.
https://github.com/kanaka/noVNC/pull/696
*** Bug 6130 has been marked as a duplicate of this bug. ***
This is fixed now.
Tested with wide web page in Firefox, and a enlarged Writer document. Scrolls up, down, left and right correctly.