Dragging sketch geometry snaps like Fusion: a dragged point (or a line dragged by its body, by its ends) snaps onto other points, line midpoints and curves, and letting go there adds the coincident, midpoint or point-on-curve constraint in the drag's undo step; it lines up horizontally and vertically with other points (dashed guides, no constraint). Snap distances in screen pixels; Ctrl or Alt held turns snapping off, also while drawing. Ring and Pt/Mid/On label on the snap. Scenario s42 (coincident and DOF, exact H alignment without a constraint, Ctrl-drag, undo, line body drag); ui.drag hold_end; synthetic drags keep their modifiers

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/ui-egui/src/control.rs

@@ -208,6 +208,10 @@ pub fn handle(app: &mut SolveApp, ctx: &egui::Context, req: &ControlRequest) ->
             for k in 1..=steps {
                 app.synthetic.push(egui::Event::PointerMoved(a + (z - a) * (k as f32 / steps as f32)));
             }
+            // `hold_end`: frames to keep the button down at the end (to look at a drag's snap).
+            for _ in 0..f("hold_end").unwrap_or(0.0).clamp(0.0, 600.0) as usize {
+                app.synthetic.push(egui::Event::PointerMoved(z));
+            }
             app.synthetic.push(egui::Event::PointerButton { pos: z, button, pressed: false, modifiers });
             ok(Value::Null)
         }