Reduce display_map snapshot creation (#39354)

Re-applies https://github.com/zed-industries/zed/pull/30840

This PR re-applies the initial
[PR](https://github.com/zed-industries/zed/pull/30840). As it was closed
because it was hard to land, because of the many conflicts. This PR
re-applies the changes for it.

In several cases we were creating multiple display_map
snapshots within the same root-level function call.
Creating a display_map snapshot is quite slow, and in some
cases we were creating the snapshot multiple times.

Release Notes:

- N/A

crates/markdown_preview/src/markdown_preview_view.rs

@@ -278,8 +278,12 @@ impl MarkdownPreviewView {
                         this.parse_markdown_from_active_editor(true, window, cx);
                     }
                     EditorEvent::SelectionsChanged { .. } => {
-                        let selection_range = editor
-                            .update(cx, |editor, cx| editor.selections.last::<usize>(cx).range());
+                        let selection_range = editor.update(cx, |editor, cx| {
+                            editor
+                                .selections
+                                .last::<usize>(&editor.display_snapshot(cx))
+                                .range()
+                        });
                         this.selected_block = this.get_block_index_under_cursor(selection_range);
                         this.list_state.scroll_to_reveal_item(this.selected_block);
                         cx.notify();