# Objective

Closes #56127

Right now there is an issue where in certain contexts, specifically GPUI
div elements, horizontal scrolling w/ a trackpad works very poorly, even
when trying to scroll sideways, vertical scrolling will happen.

This happens with both a trackpad and scrollwheel. The showcase video
also shows the broken behavior.

## Solution

The way that scrolling works in the editor is really nice, scrolls are
locked to the axis they started on but with enough force can change
axis. so by taking the ongoing scroll handler from the editor and moving
it up into GPUI, we can get that same functionality for GPUI.

I somewhat "take over" the functionality of the existing
`restrict_scroll_to_axis` property, because it seems like its purpose
was basically trying to fix this problem already, just unsuccessfully.
The alternative, and would make things overall nicer to look at, would
be to tie this to `allow_concurrent_scroll`, but the effect radius tied
to that option is way bigger, so im holding off on that atleast right
now.

## Testing

Theres a good number of scroll related tests generally, so I don't think
i've perturbed any common existing behaviors. attach the functionality
to `restrict_scroll_to_axis` really limits the surface. There are also
some new tests for the ongoing_scroll behaviors.
Tested on my mac w/ both trackpad and mouse. 

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

Video show casing all the behaviors:
broken w/ trackpad
working w/ trackpad
broken w/ scroll
working w/ scroll



https://github.com/user-attachments/assets/1935cd2d-a255-4a77-81d5-daa9f72470a6


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Release Notes:

- markdown_preview: Fix horizontal scrolling inside certain elements