G11: Color Picker and popover Fill/Stroke proxy drags (#3440)

## Summary
Illustrator lets you drag the fill colour onto the stroke (or the other way) in the Color Picker and in the fill/stroke chip popover. VectorCraft now shows the Fill/Stroke proxy pair in the Color Picker (except for artboard background edits) and applies paint.setFill or paint.setStroke when a proxy paint is dropped on either square; clicking a square switches which side the picker edits. The fill/stroke chip popover stays open while the pointer is down, while a panel paint drag is active, or when the opening click is on the chip anchor, so drags between the proxies can finish.

## Tests
- color_picker_proxy_drag_applies_fill_to_stroke (headless egui)
- existing dropping_a_proxy_paint_on_the_other_proxy_applies_it unchanged

Closes storytold/vectorcraft#3440

Co-authored-by: nrq6993-a11y <nrq6993-a11y@users.noreply.github.com>

crates/ui-egui/src/dialogs/color_picker.rs

@@ -203,6 +203,40 @@ fn proxy_color(app: &VectorcraftApp, stroke: bool) -> Color {
     }
 }
 
+/// Fill/Stroke proxies in the picker: switch which side is edited and accept drags between them.
+fn proxy_row(app: &mut VectorcraftApp, ui: &mut Ui, d: &mut Dialog) -> Option<Picked> {
+    let (fill, stroke) = crate::panels::current_paints(app);
+    let mixed = crate::panels::mixed_paints(app, ui.ctx());
+    let editing_stroke = d.bool("stroke");
+    let mut switch: Option<bool> = None;
+    ui.push_id("cp-proxy", |ui| {
+        let c = widgets::fill_stroke_proxy(ui, &fill, &stroke, mixed, !editing_stroke, 36.0);
+        if c.fill && editing_stroke {
+            switch = Some(false);
+        } else if c.stroke && !editing_stroke {
+            switch = Some(true);
+        }
+        if c.swap {
+            let _ = app.run("paint.swap", json!({}));
+        }
+        if c.default {
+            let _ = app.run("paint.default", json!({}));
+        }
+        if let Some(p) = c.dropped_fill {
+            let _ = app.run("paint.setFill", p);
+        }
+        if let Some(p) = c.dropped_stroke {
+            let _ = app.run("paint.setStroke", p);
+        }
+    });
+    if let Some(stroke) = switch {
+        d.fields.insert("stroke".into(), json!(stroke));
+        d.fields.remove(PREVIEWED);
+        return Some(Picked::of(proxy_color(app, stroke)));
+    }
+    None
+}
+
 /// `ui.colorPicker {stroke?, artboard?, color?}`: open the picker for the fill (or stroke) proxy,
 /// or for artboard `artboard`'s background color, starting from `color`, else the proxy's color
 /// or the artboard's color (white when it has none).
@@ -313,6 +347,10 @@ fn body(app: &mut VectorcraftApp, ui: &mut Ui, d: &mut Dialog) -> DialogAction {
             ui.allocate_ui_with_layout(vec2(side, 24.0), egui::Layout::right_to_left(egui::Align::Center), |ui| {
                 sample = crate::screen_color::button(app, ui);
             });
+            if !d.fields.contains_key("artboard") {
+                ui.add_space(4.0);
+                pick = pick.or(proxy_row(app, ui, d));
+            }
             ui.add_space(4.0);
             ui.with_layout(layout, |ui| {
                 ui.spacing_mut().item_spacing.x = 0.0;
@@ -1273,4 +1311,31 @@ mod tests {
             assert!(app.ui.dialog.is_none());
         }
     }
+
+    /// #3440: dragging the fill proxy onto the stroke proxy in the Color Picker applies the fill
+    /// colour to the stroke.
+    #[test]
+    fn color_picker_proxy_drag_applies_fill_to_stroke() {
+        use egui::{Event, Modifiers, PointerButton, Rect};
+        use vectorcraft_color::Paint;
+
+        use crate::widgets::PanelDrag;
+
+        let (ctx, mut app) = (context(), app());
+        app.run("paint.setFill", json!({"color": "#ff0000"})).unwrap();
+        app.run("paint.setStroke", json!({"color": "#0000ff"})).unwrap();
+        app.run("ui.colorPicker", json!({})).unwrap();
+        settled(&ctx, &mut app, screen());
+        let widget = |key: &'static str| -> Rect {
+            ctx.viewport(|vp| vp.prev_pass.widgets.layers().flat_map(|(_, w)| w.iter()).find(|w| w.id == w.parent_id.with(key)).map(|w| w.rect))
+                .expect(key)
+        };
+        let at = widget("stroke-proxy").center();
+        let red = PanelDrag::paint(Paint::solid(Color::from_hex("#ff0000").unwrap()));
+        egui::DragAndDrop::set_payload(&ctx, red);
+        let release = Event::PointerButton { pos: at, button: PointerButton::Primary, pressed: false, modifiers: Modifiers::NONE };
+        draw(&ctx, &mut app, screen(), vec![Event::PointerMoved(at), release]);
+        let stroke = app.session.shown_stroke().unwrap();
+        assert_eq!(stroke.paint.color().map(|c| c.to_hex()), Some("#ff0000".into()), "{}", app.ui.status);
+    }
 }

crates/ui-egui/src/widgets.rs

@@ -1099,10 +1099,16 @@ pub fn popover<R>(resp: &Response, toggle: bool, content: impl FnOnce(&mut Ui) -
         !was_open
     } else {
         // Popups (this one and those it opened) are on foreground layers; a click anywhere
-        // else closes it.
-        let clicked_elsewhere = ctx
-            .input(|i| i.pointer.any_click().then(|| i.pointer.interact_pos()).flatten())
-            .is_some_and(|pos| ctx.layer_id_at(pos).is_none_or(|l| l.order != egui::Order::Foreground));
+        // else closes it. While the pointer is down or a panel paint drag is active, keep the
+        // popover open so a Fill/Stroke proxy drag can finish (#3440).
+        let pointer_down = ctx.input(|i| i.pointer.any_down());
+        let panel_drag = egui::DragAndDrop::payload::<PanelDrag>(ctx).is_some();
+        let click_pos = ctx.input(|i| i.pointer.any_click().then(|| i.pointer.interact_pos()).flatten());
+        let clicked_anchor = click_pos.is_some_and(|pos| resp.rect.contains(pos));
+        let clicked_elsewhere = !pointer_down
+            && !panel_drag
+            && !clicked_anchor
+            && click_pos.is_some_and(|pos| ctx.layer_id_at(pos).is_none_or(|l| l.order != egui::Order::Foreground));
         was_open && !clicked_elsewhere
     };
     let inner =