Allow to cycle through center/top/bot scroll positions (#16134)

On top of `editor::ScrollCursorCenter`, `editor::ScrollCursorTop`,
`editor::ScrollCursorBottom` actions, adds an
`editor::ScrollCursorCenterTopBottom` one, that allows using a single
keybinding to scroll between positions on the screen.

The implementation matches a corresponding Emacs feature: there's a
timeout (1s) that is kept after every switch, to allow continuously
changing the positions, center (initial) -> top -> bottom
Scrolling behavior is the same as the existing actions (e.g. editor will
ignore scroll to bottom, if there's not enough space above).

After 1s, next position is reset to the initial, center, one.


Release Notes:

- Added an `editor::ScrollCursorCenterTopBottom` action for toggling
scroll position with a single keybinding

---------

Co-authored-by: Alex Kladov <aleksey.kladov@gmail.com>

crates/editor/src/editor_tests.rs

@@ -13149,6 +13149,77 @@ async fn test_input_text(cx: &mut gpui::TestAppContext) {
     );
 }
 
+#[gpui::test]
+async fn test_scroll_cursor_center_top_bottom(cx: &mut gpui::TestAppContext) {
+    init_test(cx, |_| {});
+
+    let mut cx = EditorTestContext::new(cx).await;
+    cx.set_state(
+        r#"let foo = 1;
+let foo = 2;
+let foo = 3;
+let fooˇ = 4;
+let foo = 5;
+let foo = 6;
+let foo = 7;
+let foo = 8;
+let foo = 9;
+let foo = 10;
+let foo = 11;
+let foo = 12;
+let foo = 13;
+let foo = 14;
+let foo = 15;"#,
+    );
+
+    cx.update_editor(|e, cx| {
+        assert_eq!(
+            e.next_scroll_position,
+            NextScrollCursorCenterTopBottom::Center,
+            "Default next scroll direction is center",
+        );
+
+        e.scroll_cursor_center_top_bottom(&ScrollCursorCenterTopBottom, cx);
+        assert_eq!(
+            e.next_scroll_position,
+            NextScrollCursorCenterTopBottom::Top,
+            "After center, next scroll direction should be top",
+        );
+
+        e.scroll_cursor_center_top_bottom(&ScrollCursorCenterTopBottom, cx);
+        assert_eq!(
+            e.next_scroll_position,
+            NextScrollCursorCenterTopBottom::Bottom,
+            "After top, next scroll direction should be bottom",
+        );
+
+        e.scroll_cursor_center_top_bottom(&ScrollCursorCenterTopBottom, cx);
+        assert_eq!(
+            e.next_scroll_position,
+            NextScrollCursorCenterTopBottom::Center,
+            "After bottom, scrolling should start over",
+        );
+
+        e.scroll_cursor_center_top_bottom(&ScrollCursorCenterTopBottom, cx);
+        assert_eq!(
+            e.next_scroll_position,
+            NextScrollCursorCenterTopBottom::Top,
+            "Scrolling continues if retriggered fast enough"
+        );
+    });
+
+    cx.executor()
+        .advance_clock(SCROLL_CENTER_TOP_BOTTOM_DEBOUNCE_TIMEOUT + Duration::from_millis(200));
+    cx.executor().run_until_parked();
+    cx.update_editor(|e, _| {
+        assert_eq!(
+            e.next_scroll_position,
+            NextScrollCursorCenterTopBottom::Center,
+            "If scrolling is not triggered fast enough, it should reset"
+        );
+    });
+}
+
 fn empty_range(row: usize, column: usize) -> Range<DisplayPoint> {
     let point = DisplayPoint::new(DisplayRow(row as u32), column as u32);
     point..point