I need to scroll in a snapshot test in my app, and kittest had no
utilities for this. Event::MouseWheel is error prone. This adds support
for some accesskit scroll actions, and uses this in kittest to add
helpers to scroll to a node / scroll the scroll area surrounding a node.
The accesskit code says down/up/left/right `Scrolls by approximately one
screen in a specific direction.`. Unfortunately it's difficult to get
the size of a "screen" (I guess that would be the size of the containing
scroll area)where I implemented the scrolling, so for now I've hardcoded
it to 100px. I think scrolling a fixed amount is still better than not
scrolling at all.
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Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>
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