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Canvas: a scroll bar press decides thumb or track where it went down, and the thumb follows the grab point (#1157)

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This commit is contained in:
Marco Arnulfo
2026-10-23 19:04:30 +02:00
parent f43d42d535
commit 9a3db3840a
1 changed files with 48 additions and 9 deletions
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crates/ui-egui/src/canvas.rs
+48 -9
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@@ -138,18 +138,31 @@ pub fn show(app: &mut SlideApp, ui: &mut Ui) {
let mut bar_active = [false; 2];
for axis in 0..2 {
let Some((track, thumb, travel)) = scroll_bar(avail, inset, overflow, offset, axis) else { continue };
let bar = ui.interact(track, scroll_id.with(("bar", axis)), Sense::click_and_drag());
let bar_id = scroll_id.with(("bar", axis));
let bar = ui.interact(track, bar_id, Sense::click_and_drag());
on_bar |= bar.hovered();
bar_active[axis] = bar.hovered() || bar.dragged();
if bar.dragged() && travel > 0.0 {
offset[axis] += bar.drag_delta()[axis] * overflow[axis] / travel;
} else if bar.clicked()
&& let Some(p) = bar.interact_pointer_pos()
&& !thumb.contains(p)
// Where the button went down and the offset then: egui calls a press a click until it
// moves 6 pt, so the release point can't tell the thumb from the track, and drag_delta
// misses those first points.
let grab_id = bar_id.with("grab");
if bar.is_pointer_button_down_on()
&& let Some(origin) = ui.input(|i| i.pointer.primary_pressed().then(|| i.pointer.press_origin()).flatten())
{
// A click on the track beside the thumb moves one window's length toward it.
let page = avail.size()[axis] * 0.9;
offset[axis] += if p[axis] < thumb.min[axis] { -page } else { page };
ui.data_mut(|d| d.insert_temp(grab_id, (origin, offset[axis])));
}
let grab: Option<(Pos2, f32)> = ui.data_mut(|d| d.get_temp(grab_id));
if let Some((origin, start)) = grab {
if bar.dragged()
&& travel > 0.0
&& let Some(p) = bar.interact_pointer_pos()
{
offset[axis] = start + (p[axis] - origin[axis]) * overflow[axis] / travel;
} else if bar.clicked() && !thumb.contains(origin) {
// A click on the track beside the thumb moves one window's length toward it.
let page = avail.size()[axis] * 0.9;
offset[axis] += if origin[axis] < thumb.min[axis] { -page } else { page };
}
}
}
if (resp.hovered() || on_bar) && !zooming {
@@ -1557,6 +1570,32 @@ mod tests {
assert!((end.max.y - (canvas.max.y - 28.0)).abs() < 2.0, "the bottom of the slide comes into view: {end:?} in {canvas:?}");
}
/// A press on the thumb that ends just past its end is a nudge, not a page: the track click
/// is decided where the button went down, not where it came up.
#[test]
fn a_short_press_on_the_thumb_does_not_page() {
let mut h = harness();
let fit = h.state().canvas_scale;
h.state_mut().ui.zoom = Some(fit * 3.0);
h.run_steps(3);
let (canvas, before) = (h.state().canvas_rect.unwrap(), h.state().slide_rect.unwrap());
let overflow = vec2((before.width() + 56.0 - canvas.width()).max(0.0), (before.height() + 56.0 - canvas.height()).max(0.0));
let offset = canvas.min + vec2(28.0, 28.0) - before.min;
let (_, thumb, _) = scroll_bar(canvas, 0.0, overflow, offset, 0).unwrap();
let (a, b) = (pos2(thumb.max.x - 3.0, canvas.max.y - 4.0), pos2(thumb.max.x + 3.0, canvas.max.y - 4.0));
h.hover_at(a);
h.run_steps(1);
h.event(Event::PointerButton { pos: a, button: egui::PointerButton::Primary, pressed: true, modifiers: Modifiers::NONE });
h.run_steps(1);
h.event(Event::PointerMoved(b));
h.run_steps(1);
h.event(Event::PointerButton { pos: b, button: egui::PointerButton::Primary, pressed: false, modifiers: Modifiers::NONE });
h.run_steps(2);
let after = h.state().slide_rect.unwrap();
let moved = before.min.x - after.min.x;
assert!((0.0..40.0).contains(&moved), "a 6 pt nudge of the thumb, not a page: {before:?} -> {after:?}");
}
/// A slide that fits has no scroll bars: a drag along the canvas edge is a marquee.
#[test]
fn a_fitted_slide_has_no_scroll_bars() {