Shape Builder: a Cmd-drag held during a merge can carry art to another tab

A Cmd-press held while a merge is computed reached the lent Selection
tool after the merge, but the drag had no document-drag origin, because
the press was handed over while Shape Builder was still the active
tool. The frontend now records the origin when the held press is made.

crates/ui-egui/src/canvas/drag.rs

@@ -413,6 +413,10 @@ pub fn apply_requests(app: &mut VectorcraftApp, r: vectorcraft_engine::Result<Ve
                     vectorcraft_engine::UiRequest::HeldPress { doc, pos, tool } => {
                         let index = doc.and_then(|uid| app.session.documents().iter().position(|d| d.uid == uid));
                         pressed_with(app, index, pos, tool);
+                        // The selection tool Cmd lent has the press now, so its drag can carry art to another tab.
+                        if let Some(ctx) = app.ui_ctx.clone() {
+                            crate::document_drag::begin(app, &ctx, pos);
+                        }
                     }
                 }
             }

crates/ui-egui/src/tests/document_drag.rs

@@ -4,6 +4,7 @@ use egui::{Event, PointerButton, Pos2};
 use serde_json::json;
 use vectorcraft_engine::Session;
 use vectorcraft_geom::Point;
+use vectorcraft_tools::{PointerEvent, PointerKind};
 
 use crate::{VectorcraftApp, canvas, chrome};
 
@@ -142,6 +143,57 @@ fn drag_fixture_modifiers(modifiers: egui::Modifiers) -> (VectorcraftApp, egui::
     (app, ctx, grab, tabs)
 }
 
+/// A Cmd-press held while a Shape Builder merge is computed reaches the lent Selection tool once
+/// the merge is applied, and its drag carries the artwork over another tab.
+#[test]
+fn a_cmd_drag_held_during_a_merge_activates_another_tab() {
+    let mut app = VectorcraftApp::new(Session::new(), Default::default());
+    app.run("file.new", json!({"width":800,"height":200,"name":"Source"})).unwrap();
+    for k in 0..40 {
+        app.run("shape.rectangle", json!({"x": k * 10, "y": 0, "width": 20, "height": 20})).unwrap();
+    }
+    app.run("select.all", json!({})).unwrap();
+    app.run("file.new", json!({"width":800,"height":200,"name":"Target"})).unwrap();
+    app.session.set_active(0);
+    app.select_tool("shapeBuilder");
+    app.ui.task_bar = false;
+    app.ui.view.rulers = false;
+    let ctx = egui::Context::default();
+    crate::theme::install_fonts(&ctx);
+    app.ui_ctx = Some(ctx.clone());
+    frame(&mut app, &ctx, 0.0, vec![]);
+    let tabs = frame(&mut app, &ctx, 0.01, vec![]);
+    let view = app.view_info();
+    app.session.pointer(&PointerEvent::new(PointerKind::Move, 15.0, 10.0), view).unwrap();
+    app.session.tool_poll(view, true).unwrap();
+    app.session.pointer(&PointerEvent::new(PointerKind::Down, 15.0, 10.0), view).unwrap();
+    app.session.pointer(&PointerEvent::new(PointerKind::Up, 15.0, 10.0), view).unwrap();
+    assert!(app.session.tool_editing(), "the click starts a merge on another thread");
+    let grab = canvas::Xf::new(app.canvas_rect.unwrap(), app.view().unwrap()).to_screen(Point::new(355.0, 10.0));
+    let modifiers = egui::Modifiers::COMMAND;
+    frame(
+        &mut app,
+        &ctx,
+        0.05,
+        vec![
+            Event::ModifiersChanged(modifiers),
+            Event::PointerMoved(grab),
+            Event::PointerButton { pos: grab, button: PointerButton::Primary, pressed: true, modifiers },
+        ],
+    );
+    assert_eq!(app.session.tool_id(), "shapeBuilder", "the press waits for the merge");
+    let r = app.session.tool_poll(view, true);
+    canvas::apply_requests(&mut app, r);
+    frame(&mut app, &ctx, 0.08, vec![]);
+    assert_eq!(app.session.tool_id(), "selection", "the held press borrowed the Selection tool");
+    frame(&mut app, &ctx, 0.1, vec![Event::PointerMoved(grab + egui::vec2(20.0, 10.0))]);
+    let tab = tabs[1].center();
+    frame(&mut app, &ctx, 0.2, vec![Event::PointerMoved(tab)]);
+    frame(&mut app, &ctx, 0.25, vec![]);
+    frame(&mut app, &ctx, 0.8, vec![]);
+    assert_eq!(app.session.active_index(), Some(1), "holding the artwork over the tab activates it");
+}
+
 /// Verify that leaving a tab restarts its dwell timer and hovering its close button never activates it.
 #[test]
 fn tab_hover_delay_restarts_after_leaving_and_close_button_does_not_activate() {