Fix panic involving growing and shrinking selections while buffer size changes (#48898)

Repro steps:

1. Make a Rust file and check it in to git
2. Delete a lot of the file
3. Expand all the diffs (cmd+")
4. Select larger syntax node (ctrl+cmd+right)
5. Collapse all the diffs
6. Select smaller syntax node (ctrl+cmd+left)
7. Panic!

The problem was we were using byte offsets instead of anchors so after
we shrank the buffer by collapsing diffs we'd be pointing past the end
of it.

Release notes:
- N/A

crates/editor/src/editor.rs

@@ -1790,7 +1790,7 @@ impl ClipboardSelection {
 
 // selections, scroll behavior, was newest selection reversed
 type SelectSyntaxNodeHistoryState = (
-    Box<[Selection<MultiBufferOffset>]>,
+    Box<[Selection<Anchor>]>,
     SelectSyntaxNodeScrollBehavior,
     bool,
 );
@@ -16590,6 +16590,10 @@ impl Editor {
             SelectSyntaxNodeScrollBehavior::CursorBottom
         };
 
+        let old_selections: Box<[Selection<Anchor>]> = old_selections
+            .iter()
+            .map(|s| s.map(|offset| buffer.anchor_before(offset)))
+            .collect();
         self.select_syntax_node_history.push((
             old_selections,
             scroll_behavior,
@@ -16612,9 +16616,15 @@ impl Editor {
                 selection.reversed = is_selection_reversed;
             }
 
+            let snapshot = self.buffer.read(cx).snapshot(cx);
+            let selections: Vec<Selection<MultiBufferOffset>> = selections
+                .iter()
+                .map(|s| s.map(|anchor| anchor.to_offset(&snapshot)))
+                .collect();
+
             self.select_syntax_node_history.disable_clearing = true;
             self.change_selections(SelectionEffects::no_scroll(), window, cx, |s| {
-                s.select(selections.to_vec());
+                s.select(selections);
             });
             self.select_syntax_node_history.disable_clearing = false;
 

crates/editor/src/editor_tests.rs

@@ -23113,6 +23113,61 @@ async fn test_toggle_deletion_hunk_at_start_of_file(
     cx.assert_state_with_diff(hunk_expanded);
 }
 
+#[gpui::test]
+async fn test_select_smaller_syntax_node_after_diff_hunk_collapse(
+    executor: BackgroundExecutor,
+    cx: &mut TestAppContext,
+) {
+    init_test(cx, |_| {});
+
+    let mut cx = EditorTestContext::new(cx).await;
+    cx.update_buffer(|buffer, cx| buffer.set_language(Some(rust_lang()), cx));
+
+    cx.set_state(
+        &r#"
+        fn main() {
+            let x = ˇ1;
+        }
+        "#
+        .unindent(),
+    );
+
+    let diff_base = r#"
+        fn removed_one() {
+            println!("this function was deleted");
+        }
+
+        fn removed_two() {
+            println!("this function was also deleted");
+        }
+
+        fn main() {
+            let x = 1;
+        }
+        "#
+    .unindent();
+    cx.set_head_text(&diff_base);
+    executor.run_until_parked();
+
+    cx.update_editor(|editor, window, cx| {
+        editor.expand_all_diff_hunks(&ExpandAllDiffHunks, window, cx);
+    });
+    executor.run_until_parked();
+
+    cx.update_editor(|editor, window, cx| {
+        editor.select_larger_syntax_node(&SelectLargerSyntaxNode, window, cx);
+    });
+
+    cx.update_editor(|editor, window, cx| {
+        editor.collapse_all_diff_hunks(&CollapseAllDiffHunks, window, cx);
+    });
+    executor.run_until_parked();
+
+    cx.update_editor(|editor, window, cx| {
+        editor.select_smaller_syntax_node(&SelectSmallerSyntaxNode, window, cx);
+    });
+}
+
 #[gpui::test]
 async fn test_expand_first_line_diff_hunk_keeps_deleted_lines_visible(
     executor: BackgroundExecutor,