feat(diff): add a button to expand all hidden lines of a diff file

[lunny]

Adds a button to each diff file header that expands all of that file's hidden (unchanged) lines in one click, and collapses them again on a second click.

This is a re-implementation of #36663, which was rejected for duplicating backend code, for OOM/DoS risk, and for fragile frontend state. The approach here is deliberately different.

Expanding: one request per gap, no new backend logic

A file's hidden lines live in "gaps" between hunks, so a file has hunks + 1 gaps — typically 1-10, not the 200 chunked requests the earlier attempt made for a 4000-line file.

BuildBlobExcerptDiffSection already had a branch that fills an entire gap in one response; only up/down do the 20-line chunking. So direction=all needs no new logic, no new option field, and no batch endpoint — it just shares that existing branch with single.

The one backend change this needs is that the offset in that branch is now derived from the hunk sizes (leftHunkSize <= 0 && rightHunkSize <= 0, i.e. "this gap runs to the end of the file") instead of from direction == "down". This is equivalent for every request the UI can emit — down is only ever offered for gaps with both hunk sizes 0, and up/single only for gaps with at least one hunk size above 0 — and it makes "how much to expand" orthogonal to "which direction the button points".

Collapsing: re-render, don't snapshot

Collapsing re-fetches the file through the existing ?file-only=true URL rather than restoring saved innerHTML. That keeps it correct when the user had already expanded some gaps by hand, and there is no DOM snapshot to go stale. The "fetch a page, take its file body" logic is now a shared helper with the existing .diff-load-button path.

Memory safety

direction=all is the only path that can materialise an unbounded number of lines, so it rejects files at or above MaxDisplayFileSize. The excerpt itself still streams through bufio.Scanner — there is no io.ReadAll of the blob anywhere.

Included bug fix

The first commit fixes a pre-existing bug: prepareDiffRenderDetail wrote the file line counts into local variables instead of the DiffRenderDetail fields that addTailSection reads, so the tail section was always built with zero indices and GetExpandDirection never reported a direction for it. The "expand down to the end of the file" button therefore never rendered for any diff. Without this, "expand all lines" could never reach a file's tail. It is a separate commit so it can be backported on its own.

Notes

• The button uses data-global-click / registerGlobalEventFunc, so it keeps working for files brought in by "load more files".
• It unfolds the file first if it is folded (vendored/generated/already-viewed), otherwise the expansion would happen invisibly — a known bug of the earlier attempt.
• It is not offered for files whose source diff is hidden behind a rendered view (CSV/TSV, SVG), for the same reason.
• The collapse URL pins both style and whitespace, since neither is persisted for anonymous visitors.

Tests

• Unit: direction=all for a leading gap, an interior gap, and an end-of-file gap, asserting exact line ranges, that no further expander row is emitted, and that highlighting still runs; plus a regression test for the tail-section fix that asserts GetExpandDirection() == "down" (verified red before the fix).
• Integration: drives a real data-expand-all-url for both the leading and the end-of-file gap, asserting the full line range through the file's last line inclusive and that no expander button is left behind; plus the 413 guard and that the guard does not affect chunked requests.
• E2E: expand, collapse back to the original row count, then fold and expand again to cover the unfold behaviour.

Screenshots

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[Copilot]

Pull request overview

Adds a per-diff-file “Expand all lines” toggle that expands every collapsed gap in that file (using one request per gap via direction=all) and collapses back by re-fetching the pristine ?file-only=true render, with backend safeguards and tests to avoid the earlier OOM/state issues.

Changes:

• Frontend: add a diff header button that expands/collapses all hidden lines in a file; reuse shared HTML-fetch parsing for “load diff” and collapse.
• Backend: support direction=all by reusing the existing full-gap fill path and add a size guard (413) for large blobs; fix tail-section line counts so “expand down” renders for tail gaps.
• Tests: unit/integration/e2e coverage for full-gap expansion (leading/middle/tail), size guard behavior, and expand/collapse/unfold UX.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
web_src/js/svg.ts	Adds new icon imports/registrations used by the expand-all toggle.
web_src/js/features/repo-diff.ts	Implements expand/collapse-all behavior and registers the global click handler.
templates/repo/diff/box.tmpl	Adds the expand-all toggle button to each diff file header (conditionally).
services/gitdiff/gitdiff.go	Adds data-expand-all-url to gap elements; fixes tail-section line count propagation.
services/gitdiff/gitdiff_excerpt.go	Adjusts full-gap excerpt fill offset semantics for direction=all/single vs end-of-file gaps.
routers/web/repo/compare.go	Rejects direction=all expansions for blobs at/above MaxDisplayFileSize (413).
services/gitdiff/gitdiff_test.go	Adds regression coverage for tail-section direction and verifies expand-all URL emission rules.
services/gitdiff/gitdiff_excerpt_test.go	Adds unit tests for direction=all on leading/middle/end-of-file gaps.
tests/integration/compare_test.go	Integration coverage for direction=all requests and the 413 guard behavior.
tests/e2e/utils.ts	Adds an API helper to update a file (used by the new e2e test).
tests/e2e/diff-expand-all.test.ts	E2E test covering expand, collapse, and unfold-then-expand behavior.
options/locale/locale_en-US.json	Adds user-facing strings for expand/collapse-all tooltip/aria labels.

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[bircni]

Please follow the contribution guidelines and add a screenshot

[lunny]

Please follow the contribution guidelines and add a screenshot

Updated