Single-line marquee tests: the dropped piece keeps its pixels (#11368)

The two no-op drag tests now drop an unmoved floating piece at the press;
check that its pixels are still where they were, not only the selection.

crates/ui-egui/src/single_line_marquee_move_tests.rs

@@ -50,6 +50,12 @@ fn steps(app: &PhotocraftApp) -> usize {
     app.session.active().unwrap().history.entries().len()
 }
 
+/// The active layer's pixel at `(x, y)`.
+fn pixel(app: &PhotocraftApp, x: i32, y: i32) -> [f32; 4] {
+    let st = app.session.active().unwrap();
+    st.doc.layer(st.active_layer.unwrap()).unwrap().surface().unwrap().rgba(x, y)
+}
+
 /// Rows 150 and 250 (New, then ⇧ adds).
 fn two_rows(app: &mut PhotocraftApp) {
     click(app, Tool::SingleRowMarquee, [300.0, 250.5], Modifiers::NONE);
@@ -173,6 +179,7 @@ fn a_drag_snapped_back_to_its_start_leaves_the_selection() {
     drag(&mut app, Tool::SingleRowMarquee, [200.0, 200.5], [200.0, 202.5], Modifiers::NONE);
     assert!(!floats(&app), "the press dropped the piece where it was (#11368)");
     assert_eq!(bounds(&app), Rect::new(100, 150, 300, 250));
+    assert_eq!(pixel(&app, 200, 200), [1.0, 0.0, 0.0, 1.0], "the dropped pixels are where they were");
 }
 
 /// A drag from inside that goes away and comes back to the press is not a click either: the
@@ -211,6 +218,7 @@ fn a_drag_that_returns_to_its_start_leaves_the_selection() {
     out_and_back(&mut app, Tool::SingleRowMarquee, [200.0, 200.5]);
     assert!(!floats(&app), "the press dropped the piece where it was (#11368)");
     assert_eq!(bounds(&app), Rect::new(100, 150, 300, 250));
+    assert_eq!(pixel(&app, 200, 200), [1.0, 0.0, 0.0, 1.0], "the dropped pixels are where they were");
 }
 
 /// A click whose release drifts under a pixel from the press (a zoomed-in hand jitter) is still a