add scrollarea

crates/core/tests/scroll_area.rs

@@ -0,0 +1,96 @@
+use gpui::{
+    Context, IntoElement, Modifiers, ParentElement as _, Render, ScrollDelta, ScrollEvent,
+    ScrollHandle, SharedString, Styled as _, TestAppContext, VisualTestContext, Window, div, point,
+    px,
+};
+use gpui_native_core::scroll_area::ScrollArea;
+use gpui_native_theme::Theme;
+
+struct Harness {
+    scroll: ScrollHandle,
+    rows: usize,
+}
+
+impl Render for Harness {
+    fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+        div().size_full().child(
+            ScrollArea::new("test-scroll")
+                .handle(self.scroll.clone())
+                .h(px(200.))
+                .w(px(300.))
+                .child(div().flex().flex_col().children((0..self.rows).map(|row| {
+                    div()
+                        .h(px(20.))
+                        .child(SharedString::from(format!("row {row}")))
+                }))),
+        )
+    }
+}
+
+/// A 300×200 scroll area over `rows` 20px rows, with the handle the test
+/// can inspect directly.
+fn scroll_area_window(rows: usize) -> (TestAppContext, VisualTestContext, ScrollHandle) {
+    let mut cx = TestAppContext::single();
+    cx.update(Theme::init);
+    let handle = ScrollHandle::new();
+    let handle_for_view = handle.clone();
+    let window = cx.add_window(move |_, _| Harness {
+        scroll: handle_for_view,
+        rows,
+    });
+    let mut visual = VisualTestContext::from_window(window.into(), &cx);
+    visual.update(|window, cx| {
+        window.draw(cx).clear(cx);
+    });
+    (cx, visual, handle)
+}
+
+#[test]
+fn wheel_scrolling_moves_the_offset_and_stays_inside_the_content() {
+    let (_cx, mut visual, handle) = scroll_area_window(30);
+
+    // 30 rows × 20px = 600px of content in a 200px viewport.
+    visual.simulate_event(ScrollEvent {
+        position: point(px(100.), px(100.)),
+        delta: ScrollDelta::Pixels(point(px(0.), px(-60.))),
+        modifiers: Modifiers::default(),
+        ..Default::default()
+    });
+
+    let offset = handle.offset().y;
+    assert_eq!(offset, px(-60.));
+
+    // Wheeling far past the end clamps to the content.
+    for _ in 0..10 {
+        visual.simulate_event(ScrollEvent {
+            position: point(px(100.), px(100.)),
+            delta: ScrollDelta::Pixels(point(px(0.), px(-60.))),
+            modifiers: Modifiers::default(),
+            ..Default::default()
+        });
+    }
+    // max_offset = 600 - 200 = 400.
+    assert_eq!(handle.offset().y, px(-400.));
+}
+
+#[test]
+fn the_content_starts_unscrolled_and_horizontal_axes_scroll_x() {
+    let (_cx, mut visual, handle) = scroll_area_window(30);
+    assert_eq!(handle.offset(), gpui::point(px(0.), px(0.)));
+
+    // A vertical-only area ignores horizontal wheel deltas.
+    visual.simulate_event(ScrollEvent {
+        position: point(px(100.), px(100.)),
+        delta: ScrollDelta::Pixels(point(px(-60.), px(0.))),
+        modifiers: Modifiers::default(),
+        ..Default::default()
+    });
+    assert_eq!(handle.offset().x, px(0.));
+}
+
+#[test]
+fn the_thumb_paints_when_the_content_overflows() {
+    let (_cx, _visual, handle) = scroll_area_window(30);
+    // 30 rows × 20px in a 200px viewport: the thumb has something to report.
+    assert!(handle.max_offset().y > px(0.));
+}