How Reverting a Change Works

This chapter explains the Revert button on each change of the Changes diff and the click on a change bar in the file editor. The user side is in Revert a Change. The diff itself is in How Diffs Work and How the inline diff works; the editor and its change bars are in How the Editor Works.

Why we need it

Staging one change was already one click, but throwing one away meant selecting its lines and running Discard Selected Lines, or editing by hand. A revert action on every change is common, and users expect it in both the diff and the editor.

How it works

The two places revert against different versions, because each shows a different comparison: the Changes diff compares the index with the working tree, and the editor's bars compare the editor's text with HEAD. So they take two paths.

flowchart TD
  A[Revert button on a diff change] --> B[revertChange.ts: the change's old and new lines]
  B --> C[DiffView: onLines discard, wholeChange]
  C --> D[changesSelection.applyLines: confirm Revert Change]
  D --> E[apply_selected_lines: version check, reverse patch on the work tree]
  E --> F[Toast with Undo: restore_discarded_lines]
  G[Click on an editor change bar] --> H[changeGutter onBarClick]
  H --> I[FileGitInfo.revertChange: confirm]
  I --> J[readHeadFile, diffLines against the text now]
  J --> K[revertLines.ts: changeAtLine, revertEdit]
  K --> L[One editor transaction, undoable]

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In the Changes diff

Side by side, DiffFrame takes onrevert and revertTitle and passes them to MergeConnector, which draws a button with the discard icon on each band, on the far side of the Stage arrow (where the merge tool's Ignore button goes). DiffEditors.changeLines(index) turns the MergeView chunk into line ranges with chunkSelection. A chunk's to is one past its last line break and may point past the end of the text, so chunkLines clamps it.

Inline, inlineView.ts adds a second button after Stage in the control gutter. Each block already carries its backend hunk, so hunkSelection reads the lines straight from it.

Both end in onLines("discard", lines, true), the Discard Selected Lines path in changesSelection.applyLines. The backend checks the diff's version, so a file that changed since the diff was shown is refused, and the Undo in the toast applies the saved patch again. wholeChange only changes the confirm's words to Revert Change. Revert is offered only in unstaged mode: a staged diff has Unstage, which already moves a change back.

In the Changes tab, a commit and a compare

These diffs are read-only, so DiffView takes onRevert (and onApply for a commit or a branch compare) instead of line actions: the same buttons side by side, and inline Apply then Revert in the control gutter. One diff view explains how GitDiffView picks the revisions; changeWorkTree (diff/diffActions.ts) confirms a Revert and calls apply_revision_lines, which patches the work tree file only, after a local history snapshot, and returns the patch for Undo.

In the editor

changeGutter takes an optional onBarClick. Its mousedown handler looks up the gutter's own markers on the clicked line, so a click beside an unchanged line or on a conflict's bar does nothing. FileGitInfo.revertChange then:

1. Confirms with a danger dialog.
2. Reads HEAD's file with readHeadFile (LF text, the path HEAD has for a staged rename).
3. Diffs it with the editor's text as it is now (diffLines), so edits made while the dialog was open count.
4. Picks the change with changeAtLine: the hunk holding the line, or the deletion whose wedge sits on it (at the end of the text, the last line).
5. Dispatches the edit from revertEdit as one transaction with userEvent: "input.revert".

Where the code lives

File	What it does
ui/web/src/lib/components/MergeConnector.svelte, DiffFrame.svelte	The Revert button on each band (UI library)
src/lib/diff/revertChange.ts	A chunk's or a hunk's lines as a LineSelection
src/lib/diff/inlineView.ts	The inline Revert button after Stage
src/lib/diff/DiffView.svelte, diffEditors.svelte.ts	Wiring the buttons to onLines
src/lib/views/changes/selection.svelte.ts	applyLines, the confirm and the Undo toast
src/lib/diff/diffActions.ts, src-tauri/src/commands/revision_lines.rs	Revert and Apply between two revisions, or a revision and the work tree
src/lib/editor/scrollMarkers.ts	changeGutter and its bar click
src/lib/editor/revertLines.ts	changeAtLine and revertEdit
src/lib/views/files/fileGitInfo.ts	revertChange for the editor

Design decisions

Reuse Discard Selected Lines in the diff. It already checks the version, keeps the patch for Undo and serializes with staging. Copying a chunk in the editors (as Stage does) would need a write of the work tree file, which the diff never does.

Edit the buffer in the editor, not the file. The editor may hold unsaved text the disk does not have. An editor transaction keeps undo, auto save and the dirty state working as for typing.

Diff again on click. The bars come from the backend and may be up to 200 ms old. Diffing HEAD with the text of that moment means the revert always matches what is there. Past MAX_EDITS the diff is one big change, so the whole range reverts.

Always confirm. Reverting loses work, and a click on a 5 pixel bar is easy to make by mistake.

Tests

• src/lib/diff/revertChange.test.ts: a chunk's and a hunk's lines, empty sides, a last line without a break.
• src/lib/editor/revertLines.test.ts: the change on a clicked line, and reverting changed, added and deleted lines at the start, middle and end.
• The backend paths are covered by the apply_selected_lines tests in commands/lines.rs and the apply_revision_lines tests in commands/revision_lines.rs: only the picked change goes back or comes in, Undo reverses it, and a staged version stays in the index.

The buttons, the hand pointer and the confirms need a check in the app. See Testing.

Bugs we fixed

The Changes tab had no Revert button.

• The issue: the Changes tab (status bar > N changes) showed changes with no way to revert one, side by side or inline.
• Why it happened: Revert came from Discard Selected Lines, which works on the index against the work tree, and the tab compares with HEAD in a read-only diff.
• The fix and why we chose it: a backend action reverts against HEAD, with the same checks, local history snapshot and Undo patch as Discard, and DiffView's onRevert shows the usual buttons in a read-only diff. It became apply_revision_lines when commits got Revert and Apply.

Commit diffs had no Revert, Apply or three columns.

• The issue: the History panel and commit tabs showed a commit's diff with no buttons, and only merge commits could show three columns, while the Changes diff had both.
• Why it happened: every place chose its own panes and buttons around DiffView and ThreePaneDiff, so features added in one place never reached the others.
• The fix and why we chose it: one GitDiffView for every place (see One diff view), with a DiffSource that decides the panes and buttons, so a new button reaches every diff at once.

Keeping this page in sync

• Update this page when the Revert buttons move, when applyLines or changeGutter change, or when the editor's bars compare with something other than HEAD.