editor: Clear previous select mode when clicking on a sticky header (#52636)

Clicking on a sticky header causes
`selections.select_ranges([anchor..anchor])` to be called, but this does
not clear the editor's `selections.select_mode()`, resulting in possible
incorrect selections if this is followed up by a shift-click. This PR
fixes that with
```diff
- selections.select_ranges([anchor..anchor]);
+ selections.clear_disjoint();
+ selections.set_pending_anchor_range(anchor..anchor, SelectMode::Character);
```
which is essentially what `editor.select(SelectPhase::Begin { ... },
...)` (i.e. a regular single click in the editor) does as well.

Before:


https://github.com/user-attachments/assets/bcf2647e-a22a-4866-8975-d29e135df148

After:


https://github.com/user-attachments/assets/fb82db51-fef1-4b7c-9954-6e076ae0b176

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed bug that caused clicking on a sticky header to not always
properly clear the previous selection.

crates/editor/src/editor_tests.rs

@@ -32309,6 +32309,91 @@ async fn test_scroll_by_clicking_sticky_header(cx: &mut TestAppContext) {
     assert_eq!(selections, vec![empty_range(4, 5)]);
 }
 
+#[gpui::test]
+async fn test_clicking_sticky_header_sets_character_select_mode(cx: &mut TestAppContext) {
+    init_test(cx, |_| {});
+    cx.update(|cx| {
+        SettingsStore::update_global(cx, |store, cx| {
+            store.update_user_settings(cx, |settings| {
+                settings.editor.sticky_scroll = Some(settings::StickyScrollContent {
+                    enabled: Some(true),
+                })
+            });
+        });
+    });
+    let mut cx = EditorTestContext::new(cx).await;
+
+    let line_height = cx.update_editor(|editor, window, cx| {
+        editor
+            .style(cx)
+            .text
+            .line_height_in_pixels(window.rem_size())
+    });
+
+    let buffer = indoc! {"
+            fn foo() {
+                let abc = 123;
+            }
+            ˇstruct Bar;
+        "};
+    cx.set_state(&buffer);
+
+    cx.update_editor(|editor, _, cx| {
+        editor
+            .buffer()
+            .read(cx)
+            .as_singleton()
+            .unwrap()
+            .update(cx, |buffer, cx| {
+                buffer.set_language(Some(rust_lang()), cx);
+            })
+    });
+
+    let text_origin_x = cx.update_editor(|editor, _, _| {
+        editor
+            .last_position_map
+            .as_ref()
+            .unwrap()
+            .text_hitbox
+            .bounds
+            .origin
+            .x
+    });
+
+    cx.update_editor(|editor, window, cx| {
+        // Double click on `struct` to select it
+        editor.begin_selection(DisplayPoint::new(DisplayRow(3), 1), false, 2, window, cx);
+        editor.end_selection(window, cx);
+
+        // Scroll down one row to make `fn foo() {` a sticky header
+        editor.scroll(gpui::Point { x: 0., y: 1. }, None, window, cx);
+    });
+    cx.run_until_parked();
+
+    // Click at the start of the `fn foo() {` sticky header
+    cx.simulate_click(
+        gpui::Point {
+            x: text_origin_x,
+            y: 0.5 * line_height,
+        },
+        Modifiers::none(),
+    );
+    cx.run_until_parked();
+
+    // Shift-click at the end of `fn foo() {` to select the whole row
+    cx.update_editor(|editor, window, cx| {
+        editor.extend_selection(DisplayPoint::new(DisplayRow(0), 10), 1, window, cx);
+        editor.end_selection(window, cx);
+    });
+    cx.run_until_parked();
+
+    let selections = cx.update_editor(|editor, _, cx| display_ranges(editor, cx));
+    assert_eq!(
+        selections,
+        vec![DisplayPoint::new(DisplayRow(0), 0)..DisplayPoint::new(DisplayRow(0), 10)]
+    );
+}
+
 #[gpui::test]
 async fn test_next_prev_reference(cx: &mut TestAppContext) {
     const CYCLE_POSITIONS: &[&'static str] = &[

crates/editor/src/element.rs

@@ -6732,7 +6732,13 @@ impl EditorElement {
                             SelectionEffects::scroll(Autoscroll::top_relative(line_index)),
                             window,
                             cx,
-                            |selections| selections.select_ranges([anchor..anchor]),
+                            |selections| {
+                                selections.clear_disjoint();
+                                selections.set_pending_anchor_range(
+                                    anchor..anchor,
+                                    crate::SelectMode::Character,
+                                );
+                            },
                         );
                         cx.stop_propagation();
                     });