outline_panel: Show deleted files inside the file tree (#63570)

# Objective

In a branch diff — or any multi-buffer containing deleted files —
deleted files render as flat "external file" rows above the outline
panel's tree, instead of at the path they used to occupy. They lose all
directory context and appear as a bare list of filenames detached from
the tree, which is confusing precisely when you are trying to review
what a branch changed.

The cause is that `File::project_entry_id` returns `None` for
`DiskState::Deleted`, while the outline panel's tree is keyed end-to-end
by `ProjectEntryId` — depth map, collapse state, children counts. A
deleted buffer therefore falls through to the external-file path in
`update_fs_entries`, even though its `File` still carries a perfectly
good worktree and path.

## Solution

Synthesize the missing entries at the point of classification and let
the existing pipeline run unchanged.

- Entries are synthesized for the deleted file **and** for any ancestor
directories that no longer exist, so a wholly-deleted folder still
appears nested at its old location. An ancestor is only taken from the
worktree if it is a directory; a path that is a file now but was a
directory when the deleted file existed is synthesized too, and sorted
after the file so the nesting pass keeps the directory on its stack.
- Ids come from a per-panel registry that hands them out sequentially
downward from just below `ProjectEntryId::MAX`, keyed by path and kind.
Real ids count up from zero, so the ranges cannot meet; sequential
allocation means two entries can never share an id (the entry maps are
keyed by id alone, so a collision would drop a row); and the registry is
kept across updates, so collapse state on synthesized rows survives.
- Whether a buffer with no worktree entry is a deletion is decided by
its `DiskState`: `Deleted` always, `Present` never, `Historic` by its
own flag, and `New` only when git status reports a deletion or the
excerpt's diff has a base text. The last case is a file deleted in a
commit on the branch: it is absent from disk, the working-tree status
says nothing about it, and `GitStore::display_status` only reflects
branch-diff statuses when `git.diff_base` is `default_branch`. A
deletion has a base version; a path that never existed has none. A
scan-excluded file, which also has no entry, is `Present` and stays an
external row.
- Auto-fold stops where deletion state changes, so a surviving `parent/`
is never struck through together with its deleted child, and
`select_parent` treats a folded row as a valid parent of the folded row
beneath it.
- Auto-reveal resolves a deleted row's ancestors from the panel's own
`fs_entries` rather than re-querying the worktree, and only real
directories are ever passed to `Project::expand_entry` (a synthesized id
would go out as an `ExpandProjectEntry` a remote host rejects). Opening
a deleted row resolves its excerpt from the buffer id, since
`path_for_entry` cannot resolve a synthesized entry.

Deleted rows are struck through by a `line_through` on the row's label
wrapper. `HighlightedLabel` builds its text runs from
`window.text_style()`, so an ancestor's style reaches them even though
`LabelLike`'s own `strikethrough()` does not; nothing outside
`crates/outline_panel` changes. Colour is deliberately left to git
status, matching deleted editor tabs — a committed deletion leaves the
working tree clean, so strikethrough is the only signal in a branch
diff.

## Testing

Thirteen new tests, each asserting the rendered tree through
`display_entries`, which now prints external files and marks deleted
rows:

- deleted file nests under its surviving parent; deleted file inside a
wholly-deleted directory gets synthesized ancestors at the right depth
- a file absent from disk and from git status nests when its excerpt
carries a base text, while a buffer on a path that never existed stays
an external file, unmarked
- a modified, scan-excluded tracked file stays an external file,
unmarked
- same-path file + synthesized directory: nesting and auto-reveal both
pick the directory
- auto-fold stops at the deletion boundary; `select_parent` climbs from
a folded deleted chain to the folded parent above it
- opening a deleted row scrolls the editor to its excerpt
- auto-reveal expands the ancestors of a deleted file and ignores an
external file on a nonexistent path
- buffers with no `File` still become `ExternalFile`
- synthetic ids are distinct per path and kind, and stable

`./script/clippy`, `cargo fmt --all -- --check`, `cargo test -p
outline_panel` (33) are green. Tested manually on macOS against a
fixture branch with a committed file deletion, a wholly-deleted
directory under a surviving empty one, a directory replaced by a
same-path file, an uncommitted deletion, a scan-excluded modified file,
and a modified and an added file.

## Suggested .rules additions

- `GitStore::display_status` only reflects branch-diff statuses when
`git.diff_base` is `default_branch`; the branch diff view can be open
while the setting is `head`. Do not use it to detect committed deletions
— a commit-deleted file's buffer is `DiskState::New`; check the
excerpt's diff for a base text instead.
- Outline panel: a directory with mixed children is marked unfolded
while `fs_entries` is built, so an auto-fold chain only forms through
single-subdirectory links. Tests for folding must use that shape.
- `HighlightedLabel` builds its runs from `window.text_style()` at
render time, so `LabelLike` modifiers such as `strikethrough()` and
`italic()` are no-ops on it; put the style on an ancestor element
instead.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

The same branch diff, which deletes `src/utils/format.rs` (its directory
survives) and the whole of `src/legacy/` (including a nested
`internal/`), alongside a modified and an added file.

| Before | After |
| --- | --- |
| <img
src="https://github.com/user-attachments/assets/bf503759-77a7-4f31-80bb-a85936de0b8a"
width="300"> | <img
src="https://github.com/user-attachments/assets/c558b94b-4ca5-447f-a049-a3ab280c679a"
width="300"> |
| Deleted files sit above the project root as bare filenames, detached
from the tree | Each is nested at its original path, with `legacy/` and
`internal/` synthesized since they no longer exist on disk |

Note that `utils/` renders normally while `legacy/` and `internal/` are
struck through: the marking is per-entry, not applied to every ancestor
on a deleted file's path.

---

Release Notes:

- Fixed deleted files appearing outside the file tree in the outline
panel when viewing a diff

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

docs/src/outline-panel.md

@@ -13,6 +13,12 @@ When viewing a "singleton" buffer (i.e., a single file on a tab), the outline pa
 
 ## Usage with multibuffers
 
+File rows follow the order of the files in the multibuffer.
+If files from a folder appear in separate parts of the multibuffer, the outline panel repeats that folder's path group to preserve their order.
+
+When a multibuffer includes deleted project files, they remain grouped under their original paths with struck-through file names.
+Folder rows group files by path and are not struck through, even when the folders no longer exist on disk.
+
 The outline panel truly excels when used with multi-buffers. Here are some examples of its versatility:
 
 ### Project Search Results