editor: Fix semantic tokens missing when opening buffer from multibuffer

[Dnreikronos]

Summary

Semantic token highlighting was missing when opening a file from multibuffer search results (Ctrl+Shift+F). Which file got hit depended on window size and scroll offset.

Root cause

Two async tasks race to write post_scroll_update:

1. set_visible_line_count (scroll.rs:682) fires on first render and spawns a task that calls register_visible_buffers + update_lsp_data (requests semantic tokens).

2. open_buffers_in_workspace (editor.rs:25049) calls change_selections with autoscroll right after creating the editor. This emits ScrollPositionChanged, whose handler (editor.rs:2655) replaces post_scroll_update with a task calling update_data_on_scroll.

3. update_data_on_scroll (editor.rs:26099) has a singleton guard: if !self.buffer().read(cx).is_singleton() that skips update_lsp_data for single-file buffers. This is a scroll optimization, singleton buffers don't change their visible buffer set on scroll.

4. The initial task gets dropped, the replacement skips update_lsp_data, semantic tokens are never requested.

Fix

Added a needs_initial_lsp_data flag to the Editor struct, set to true on creation. update_data_on_scroll checks this flag alongside the singleton guard, so update_lsp_data runs at least once even for singletons. The flag flips to false right after, so subsequent scrolls behave exactly as before. No perf impact after the first render.

Self-review checklist

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Closes #53051

Demo

Before:
https://github.com/user-attachments/assets/77d07d95-cb4a-44ff-842d-1f7a46653ca9
After:

semantic.mp4

Release notes

• Fixed semantic token highlighting missing when opening a buffer from multibuffer search results