Field scrub: the field always senses drags; Alt as the drag begins makes it a scrub until the button is let go

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/ui-egui/src/field_tests.rs

@@ -1216,4 +1216,17 @@ fn alt_held_over_a_field_scrubs_it() {
     drag(&mut u, at, 25.0, Modifiers::NONE);
     assert_eq!(geom(&u, a).0, 35.0, "without Alt the field doesn't scrub");
     assert_eq!(undo_steps(&u), steps + 1);
+    // Alt and the press in the same frame still scrub, and Alt let go before the button still
+    // ends the scrub with its undo step.
+    let (at, _) = field(&mut u, "X");
+    let press = |pressed: bool, m: Modifiers, p: Pos2| Event::PointerButton { pos: p, button: egui::PointerButton::Primary, pressed, modifiers: m };
+    u.frame_with(vec![Event::PointerMoved(at), press(true, alt, at)], alt);
+    for k in 1..=10 {
+        u.frame_with(vec![Event::PointerMoved(pos2(at.x + 2.0 * k as f32, at.y))], alt);
+    }
+    let to = pos2(at.x + 20.0, at.y);
+    u.frame_with(vec![Event::PointerMoved(to), press(false, Modifiers::NONE, to)], Modifiers::NONE);
+    u.frame(vec![]);
+    assert_eq!(geom(&u, a).0, 55.0, "twenty more");
+    assert_eq!(undo_steps(&u), steps + 2, "committed when the button went up, without Alt");
 }

crates/ui-egui/src/widgets.rs

@@ -268,10 +268,16 @@ fn number_field_impl(
     let mut editing: Option<String> = ui.data(|d| d.get_temp(editing_id));
     // Alt held over the field scrubs it too, as the label always does: "hover the input field
     // itself and hold down the modifier key" (Adjust alignment, rotation, position, and
-    // dimensions). The drag sense stays on until a scrub begun that way is let go.
+    // dimensions). The field always senses drags (egui assigns a press by the frame before it, so
+    // a sense that waits for Alt would miss a press that arrives with it); Alt as the drag begins
+    // is what makes it a scrub, and that holds until the button is let go, Alt released or not.
     let field_id = id.with("field");
-    let alt_scrub = editing.is_none() && (ui.input(|i| i.modifiers.alt) || ui.ctx().is_being_dragged(field_id));
-    let field_resp = ui.interact(edit_rect, field_id, if alt_scrub { Sense::CLICK | Sense::DRAG } else { Sense::click() });
+    let field_resp = ui.interact(edit_rect, field_id, Sense::click_and_drag());
+    let alt_now = ui.input(|i| i.modifiers.alt);
+    let scrubbing_id = id.with("altscrub");
+    let alt_scrub =
+        editing.is_none() && if field_resp.drag_started() { alt_now } else { ui.data(|d| d.get_temp::<bool>(scrubbing_id)).unwrap_or(false) };
+    ui.data_mut(|d| d.insert_temp(scrubbing_id, alt_scrub && field_resp.dragged() && !field_resp.drag_stopped()));
     a11y::number(&field_resp, lead.name(), value, suffix, empty.text());
     a11y::ring_around(ui, rect, &field_resp);
     // The label scrubs with the pointer; the keyboard stops at the value only.
@@ -297,7 +303,7 @@ fn number_field_impl(
         }
         Lead::None | Lead::Named(_) => {}
     }
-    if lead_resp.hovered() || lead_resp.dragged() || (alt_scrub && (field_resp.hovered() || field_resp.dragged())) {
+    if lead_resp.hovered() || lead_resp.dragged() || (alt_now && editing.is_none() && field_resp.hovered()) || (alt_scrub && field_resp.dragged()) {
         ui.ctx().set_cursor_icon(egui::CursorIcon::ResizeHorizontal);
     }
     // On the field, Alt is what makes the drag a scrub, so it is not the fine rate there.
@@ -349,7 +355,7 @@ fn number_field_impl(
     }
     // Typing on a field the keyboard reached (Tab) edits it: the editor takes the typed text.
     let typed_now = editing.is_none() && on_field && ui.input(|i| i.events.iter().any(|e| matches!(e, egui::Event::Text(_))));
-    let started = editing.is_none() && ((field_resp.clicked() && !alt_scrub) || typed_now);
+    let started = editing.is_none() && ((field_resp.clicked() && !alt_now) || typed_now);
     // The text the field would commit to no effect: what it showed when the edit began, or the
     // value the arrows last stepped it to.
     let kept_id = id.with("kept");