editor: Clear previous select mode when clicking on a sticky header

[timvermeulen]

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

- 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:

before.mov

After:

after.mov

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

Release Notes:

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