Window: move only by dragging title bar (#8183)

* Part of https://github.com/emilk/egui/issues/8180

So far, you've been able to move any `egui::Window` by dragging anywhere
on it. This makes sense on touch screens with thick fingers, but less so
on non-touch-screens.

With this PR, you can now control it with a new enum `WindowDrag`

tests/egui_tests/tests/test_window_drag.rs

@@ -0,0 +1,195 @@
+//! Tests for [`Window::drag_area`] and [`Window::movable`].
+//!
+//! Each test sets up a window with a particular drag configuration, drags
+//! either inside or outside the title bar, and asserts on the area rect's
+//! delta. `WindowDrag::OnTouch` is not exercised here since it just resolves
+//! to `TitleBar` (no touch screen in headless tests).
+
+use egui::{Id, Pos2, Sense, Vec2, Window, WindowDrag};
+use egui_kittest::Harness;
+
+struct State {
+    drag_area: WindowDrag,
+    movable: bool,
+}
+
+fn build(state: State) -> Harness<'static, State> {
+    let mut harness = Harness::builder()
+        .with_size(Vec2::new(500.0, 400.0))
+        .with_max_steps(40) // Area requests a repaint every frame while pressed.
+        .build_ui_state(
+            move |ui, state: &mut State| {
+                Window::new("test_win")
+                    .id(Id::new("test_win"))
+                    .drag_area(state.drag_area)
+                    .movable(state.movable)
+                    .default_pos([100.0, 80.0])
+                    .default_size([180.0, 140.0])
+                    .show(ui.ctx(), |ui| {
+                        // A passive widget fills the body; it has no drag sense
+                        // of its own, so the Area / title-bar widget is what
+                        // decides whether a drag moves the window.
+                        ui.allocate_response(ui.available_size(), Sense::hover());
+                    });
+            },
+            state,
+        );
+    // Let the window settle (auto-position / size, then idle).
+    harness.run_steps(4);
+    harness
+}
+
+fn window_rect(harness: &Harness<'_, State>) -> egui::Rect {
+    egui::AreaState::load(&harness.ctx, Id::new("test_win"))
+        .expect("window area should be persisted after the first frame")
+        .rect()
+}
+
+/// Drag the pointer from `from` to `to` over multiple frames; release at the end.
+fn drag(harness: &mut Harness<'_, State>, from: Pos2, to: Pos2) {
+    harness.drag_at(from);
+    harness.run_steps(4);
+    harness.hover_at(to);
+    harness.run_steps(4);
+    harness.drop_at(to);
+    harness.run_steps(4);
+}
+
+fn titlebar_pos(rect: egui::Rect) -> Pos2 {
+    // Just inside the title bar:
+    Pos2::new(rect.center().x, rect.top() + 8.0)
+}
+
+fn body_pos(rect: egui::Rect) -> Pos2 {
+    // Well below the title bar:
+    Pos2::new(rect.center().x, rect.bottom() - 30.0)
+}
+
+#[test]
+fn title_bar_drag_on_titlebar_moves_window() {
+    let mut harness = build(State {
+        drag_area: WindowDrag::TitleBar,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = titlebar_pos(before);
+    let to = from + Vec2::new(60.0, 40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        20.0 < moved.x && 20.0 < moved.y,
+        "TitleBar + drag on titlebar should move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn title_bar_drag_outside_titlebar_keeps_window_put() {
+    let mut harness = build(State {
+        drag_area: WindowDrag::TitleBar,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = body_pos(before);
+    let to = from + Vec2::new(60.0, -40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        moved.length() < 1.0,
+        "TitleBar + drag in the body should NOT move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn anywhere_drag_in_body_moves_window() {
+    let mut harness = build(State {
+        drag_area: WindowDrag::Anywhere,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = body_pos(before);
+    let to = from + Vec2::new(60.0, -40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        20.0 < moved.x && moved.y < -20.0,
+        "Anywhere + drag anywhere should move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn movable_false_keeps_window_put_even_on_titlebar() {
+    // Regression: a `movable(false)` window used to still move when the user
+    // dragged the title bar in `TitleBar` mode.
+    let mut harness = build(State {
+        drag_area: WindowDrag::TitleBar,
+        movable: false,
+    });
+
+    let before = window_rect(&harness);
+    let from = titlebar_pos(before);
+    let to = from + Vec2::new(60.0, 40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        moved.length() < 1.0,
+        "TitleBar + movable(false) should NOT move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn off_keeps_window_put_on_body_drag() {
+    // `WindowDrag::Off` should freeze the window regardless of `movable`.
+    let mut harness = build(State {
+        drag_area: WindowDrag::Off,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = body_pos(before);
+    let to = from + Vec2::new(60.0, -40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        moved.length() < 1.0,
+        "Off + drag in the body should NOT move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn off_keeps_window_put_on_titlebar_drag() {
+    let mut harness = build(State {
+        drag_area: WindowDrag::Off,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = titlebar_pos(before);
+    let to = from + Vec2::new(60.0, 40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        moved.length() < 1.0,
+        "Off + drag on titlebar should NOT move the window (delta = {moved:?})"
+    );
+}