outline_panel: Add setting to hide symbols in multi-buffer views

[FrantisekGazo]

Closes #60242

Objective

When reviewing a branch diff — or project search results — the outline panel lists every affected excerpt and symbol beneath each file. On a diff touching more than a handful of files, this consumes so much vertical space that you cannot see the list of changed files at all, which is usually the thing you want the panel for.

Existing settings don't cover this:

• expand_outlines_with_depth: 0 still renders excerpt rows and top-level symbol rows.
• Collapsing a file row in the panel is coupled to folding that buffer in the editor, so it can't be used purely to tidy the panel.

Solution

A display-only mode that reduces multi-buffer views to just the directory/file tree.

• New outline_panel.multi_buffer_hide_symbols setting (default false). When enabled, multi-buffer views render only directory, file and folded-directory rows — excerpt, symbol and search-match rows are hidden.
• New outline panel: toggle symbols action, plus a button in the panel footer, which toggles an in-memory per-panel override of the setting (the same pattern as editor: toggle inlay hints). Changing the setting clears the override, so the setting wins.
• Single-file views are exempt, and the footer button is not shown there — the panel contains only symbols in that case, so hiding them would leave it empty.
• The filter is display-only: collapsed_entries and editor buffer folding are never touched.
• Outline fetching is skipped while the mode is active, so large diffs don't pay for outlines that will not be rendered. Turning the mode off re-triggers fetching.

This adds one new icon, file_code_off, used by the footer button when symbols are hidden. cc @zed-industries/design for review per the icon guidelines — it is file_code with the slash and outline gaps from file_ignored, keeping the parts of the chevrons that fall on the near side of the slash.

Testing

Three new tests in crates/outline_panel/src/outline_panel.rs:

• test_hide_symbols_in_multibuffer — outline rows are present by default; only Fs/FoldedDirs entries remain once the mode is enabled via the setting (which also exercises the settings observer) and via the action; outline rows return when it is toggled off, confirming outlines are re-fetched; asserts collapsed_entries is unchanged at every step, covering the display-only guarantee.
• test_hide_symbols_exempts_singleton — a single-file view still renders its outline with the setting enabled.
• test_hide_symbols_hides_search_matches — project search match rows are hidden while files and directories still render, and return when toggled off.

cargo test -p outline_panel (15 passing), cargo test -p icons, cargo test -p settings_ui -p settings_content, and ./script/clippy on the touched crates are all green.

Tested manually on macOS against a branch diff, project search results, and single-file buffers, checking both the setting and the action (from the panel footer button and from the command palette with focus outside the panel), and that expanding/collapsing directories and editor folding still behave as before.

Self-Review Checklist:

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

Showcase

	Symbols shown (default)	Symbols hidden
Footer toggle
Branch diff
Project search

Single-file views are unaffected — symbols still render, and the toggle is not offered:

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Release Notes:

• Fixed too many outlines shown per excerpt in the outline panel. Added an outline_panel.multi_buffer_hide_symbols setting and an outline panel: toggle symbols action to show only files and directories in the outline panel for multi-buffer views.

[SomeoneToIgnore]

Thanks, nice idea!

Took a liberty to push a fix for incorrect outline filtering in excerpts; and had to fix the focus issues: removing the symbols did not re-select anything in the pane, spoiling its uninterrupted navigation ergonomics.