Canvas · the wheel scrolls as scroll boxes do; pans hold at the press

A wheel without a modifier now slides the content by its delta, as
gpui's scroll boxes do; it moved the other way. The wheel reads the
live viewport and skips an end with no delta, and a drag pans from the
press by the pointer's whole way with the viewport held at the press,
so events that land in one frame no longer overwrite each other. Tests:
a plain wheel moves the content up with it; three wheels and an empty
end add up, and four moves pan their sum; a small frame fits at a zoom
of one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

TASKS.md

@@ -373,3 +373,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T31c | 2 | PASS | |
 | T31d | 1 | FAIL | a focused segment past the strip's fold stayed hidden; the tracker lost focus as Start and Stop swapped; Stop and Skip went untested; percent did not refuse content |
 | T31d | 2 | PASS | |
+| T32a | 1 | FAIL | the plain wheel panned against every scroll box; the handlers panned from the render's viewport, so events within a frame overwrote each other; the fit's cap at one went untested |

src/canvas/plane.rs

@@ -22,11 +22,11 @@ struct Pan {
     owner: EntityId,
 }
 
-/// The canvas's box and the pointer of a pan under way.
+/// The canvas's box, and the pointer and viewport at the press of a pan under way.
 #[derive(Default)]
 struct Plane {
     bounds: Bounds<Pixels>,
-    grip: Option<Point<Pixels>>,
+    grip: Option<(Point<Pixels>, Viewport)>,
 }
 
 /// The view pixels of a canvas frame.
@@ -400,6 +400,7 @@ impl RenderOnce for InfiniteCanvas {
         let owner = plane.entity_id();
         let (wheel, pan, drop) = (set.clone(), set, plane.clone());
         let (scrolled, dragged, pressed) = (plane.clone(), plane.clone(), plane);
+        let (live, held) = (local.clone(), local.clone());
         div()
             .id(id)
             .relative()
@@ -416,34 +417,37 @@ impl RenderOnce for InfiniteCanvas {
                         (lines.x * line, lines.y * line)
                     }
                 };
+                if delta == (0.0, 0.0) {
+                    return;
+                }
                 let pointer = event.position - bounds.origin;
                 let about = (f32::from(pointer.x), f32::from(pointer.y));
-                let now = view;
+                let now = live.read(cx).value;
                 let next = match event.modifiers.platform || event.modifiers.control {
                     true => now.zoomed((-delta.1 / 240.0).exp(), about),
-                    false => now.panned((-delta.0, -delta.1)),
+                    false => now.panned(delta),
                 };
                 cx.stop_propagation();
                 wheel(next, window, cx);
             })
             .on_mouse_down(gpui::MouseButton::Left, move |event, _, cx| {
-                pressed.update(cx, |plane, _| plane.grip = Some(event.position))
+                let now = held.read(cx).value;
+                pressed.update(cx, |plane, _| plane.grip = Some((event.position, now)))
             })
             .on_drag(Pan { owner }, |_, _, _, cx| cx.new(|_| EmptyView))
             .on_drag_move::<Pan>(move |event, window, cx| {
                 if event.drag(cx).owner != owner {
                     return;
                 }
-                let pointer = event.event.position;
-                let last = dragged.update(cx, |plane, _| plane.grip.replace(pointer));
-                if let Some(last) = last {
-                    let delta = pointer - last;
-                    pan(
-                        view.panned((f32::from(delta.x), f32::from(delta.y))),
-                        window,
-                        cx,
-                    );
-                }
+                let Some((press, start)) = dragged.read(cx).grip else {
+                    return;
+                };
+                let way = event.event.position - press;
+                pan(
+                    start.panned((f32::from(way.x), f32::from(way.y))),
+                    window,
+                    cx,
+                );
             })
             .on_drop(move |_: &Pan, _, cx| drop.update(cx, |plane, _| plane.grip = None))
             .child(painted)

src/canvas/tests.rs

@@ -155,3 +155,53 @@ fn a_press_on_the_map_centers_the_view_there(cx: &mut TestAppContext) {
         "the view centers on the point pressed: {middle:?}"
     );
 }
+
+fn wheel(at: (f32, f32), dy: f32, modifiers: Modifiers) -> ScrollWheelEvent {
+    ScrollWheelEvent {
+        position: point(px(at.0), px(at.1)),
+        delta: ScrollDelta::Pixels(point(px(0.0), px(dy))),
+        modifiers,
+        touch_phase: TouchPhase::Moved,
+    }
+}
+
+#[gpui::test]
+fn a_plain_wheel_moves_the_content_as_a_scroll_box_does(cx: &mut TestAppContext) {
+    let (host, cx) = stage(plane, cx);
+    cx.simulate_event(wheel((100.0, 50.0), -50.0, Modifiers::none()));
+    settle(cx);
+    let view = *heard(&host, cx).last().expect("a scroll");
+    assert_eq!(
+        view.to_view((0.0, 0.0)),
+        (0.0, -50.0),
+        "the content went up with the wheel"
+    );
+}
+
+#[gpui::test]
+fn wheels_and_moves_add_up(cx: &mut TestAppContext) {
+    let (host, cx) = stage(plane, cx);
+    for dy in [-10.0, -10.0, -10.0, 0.0] {
+        cx.simulate_event(wheel((100.0, 50.0), dy, Modifiers::none()));
+    }
+    settle(cx);
+    let view = *heard(&host, cx).last().expect("a scroll");
+    assert_eq!(
+        view.y, 30.0,
+        "three wheels add up, and an end that moves nothing leaves it"
+    );
+    let at = |x: f32, y: f32| point(px(x), px(y));
+    cx.simulate_mouse_move(at(200.0, 150.0), None, Modifiers::none());
+    cx.simulate_mouse_down(at(200.0, 150.0), MouseButton::Left, Modifiers::none());
+    for x in [190.0, 180.0, 170.0, 160.0] {
+        cx.simulate_mouse_move(at(x, 150.0), Some(MouseButton::Left), Modifiers::none());
+    }
+    cx.simulate_mouse_up(at(160.0, 150.0), MouseButton::Left, Modifiers::none());
+    settle(cx);
+    let view = *heard(&host, cx).last().expect("a pan");
+    assert_eq!(
+        (view.x, view.y),
+        (40.0, 30.0),
+        "the drag pans from the press by the pointer's whole way"
+    );
+}

src/canvas/view.rs

@@ -159,6 +159,9 @@ mod tests {
         let view = Viewport::fitting(Frame::new(0.0, 0.0, 800.0, 400.0), (440.0, 440.0), 20.0);
         assert_eq!(view.zoom, 0.5);
         assert_eq!(view.to_view((400.0, 200.0)), (220.0, 220.0));
+        let small = Viewport::fitting(Frame::new(0.0, 0.0, 100.0, 50.0), (440.0, 440.0), 20.0);
+        assert_eq!(small.zoom, 1.0, "no nearer than a zoom of one");
+        assert_eq!(small.to_view((50.0, 25.0)), (220.0, 220.0));
     }
 
     #[test]