Update ScrollArea drag velocity when drag stopped #5175
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Fixes emilk#5174. The drag velocity was not being updated unless the cursor counted as "dragging", which only happens when it's in motion. This effectively guarantees that the drag velocity will never be zero, even if the cursor is not moving, and results in spurious scroll velocity being applied when the cursor is released. Instead, we update the velocity only when the drag is stopped, which is when the kinetic scrolling actually needs to begin.
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Preview available at https://egui-pr-preview.github.io/pr/5175-fix-drag-velocity |
emilk
approved these changes
Sep 30, 2024
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| for d in 0..2 { | ||
| if scroll_enabled[d] { | ||
| ui.input(|input| { | ||
| state.vel[d] = input.pointer.velocity()[d]; | ||
| }); | ||
| } else { | ||
| state.vel[d] = 0.0; | ||
| } | ||
| } |
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It would be nice if we could write this as
Suggested change
| for d in 0..2 { | |
| if scroll_enabled[d] { | |
| ui.input(|input| { | |
| state.vel[d] = input.pointer.velocity()[d]; | |
| }); | |
| } else { | |
| state.vel[d] = 0.0; | |
| } | |
| } | |
| state.vel = scroll_enabled.to_vec2() * input.pointer.velocity(); |
(requires implementing adding a fn to_vec2(&self) to Vec2b)
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Fixes #5174.
The drag velocity was not being updated unless the cursor counted as "dragging", which only happens when it's in motion. This effectively guarantees that the drag velocity will never be zero, even if the cursor is not moving, and results in spurious scroll velocity being applied when the cursor is released.
Instead, we update the velocity only when the drag is stopped, which is when the kinetic scrolling actually needs to begin. Note that we immediately apply the scroll velocity on the same frame that we first set it, to avoid a 1-frame gap where the scroll area doesn't move.
I believe that not setting
scroll_stuck_to_endandoffset_targetwhen the drag is released is the correct thing to do, as they should apply immediately once the user stops dragging. Should we maybe clear the drag velocity instead ifscroll_stuck_to_endis true oroffset_targetexists?