Add "view changes" action into PR webview header

[Alex Ross (alexr00)]

No description provided.

[Copilot]

Copilot review overview

🟡 Changes recommended

Unresolved moderate issues affect missing-head handling, merged-PR diff correctness, and error reporting.

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Review tier: Lite
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​github/​pullRequestOverview.ts — This handler has no failure path. If the provider is unavailable, the PR has no head/base commit,…

What changed in this PR

Adds an agent-session-only “View Changes” action to PR webview headers, opening read-only diffs from local Git or GitHub.

Changes:

• Adds regular and sticky header actions with a diff icon.
• Implements webview messaging and local/GitHub diff construction.
• Extends view state and adds UI/model tests.

File	Change	Final review notes
webviews/​editorWebview/​test/​overview.test.tsx	Tests header visibility and clicks.	—
webviews/​editorWebview/​test/​builder/​pullRequest.ts	Updates PR test fixtures.	—
webviews/​components/​stickyHeader.tsx	Adds the View Changes action.	—
webviews/​components/​icon.tsx	Registers the diff icon.	—
webviews/​components/​header.tsx	Adds the View Changes action.	moderate (1 vote): Hide or disable it when the head repository or ref is unavailable.
webviews/​common/​context.tsx	Sends the view-changes command.	—
src/​test/​github/​pullRequestModel.test.ts	Tests diff sources and URI construction.	nit (1 vote): Add mocked GitHub coverage for merged and open-PR fallback paths.
src/​github/​views.ts	Extends PR view state.	—
src/​github/​pullRequestOverview.ts	Handles the view-changes command.	moderate (2 votes): Add failure handling so requests receive _throwError when operations fail.
src/​github/​pullRequestModel.ts	Builds local and GitHub-backed diffs.	moderate (1 vote): Handle deleted PR heads without throwing. moderate (1 vote): Avoid the local fast path for merged PRs unless using the verified original base.
resources/​icons/​codicons/​diff-multiple.svg	Adds the diff icon asset.	—

Suppressed comments (4)

src/github/pullRequestModel.ts:1647

• A deleted PR head can still have headRefOid while headRepository is absent, in which case parseRef leaves PullRequestModel.head undefined. The header still renders this action for agent workspaces, so this guard throws before the GitHub file-list fallback and makes “View Changes” unusable for those PRs. Preserve/use the raw head OID or suppress the action when no head commit can be resolved.

		const headCommit = pullRequestModel.head?.sha;

src/github/pullRequestModel.ts:1657

• The merged-PR branch currently enables the local fast path by diffing base.sha against head.sha. For a merged PR, base.sha can be the current base-branch tip (including the merge commit), so that range contains the wrong changes; the existing getRawFileChangesInfo deliberately uses the PR file-list API for merged PRs instead. Skip the local fast path for merged PRs and let getAllFileChangesInfo() supply the PR's file set/base, or use a verified original-base commit.

		let startCommit = pullRequestModel.item.merged
			? pullRequestModel.base.sha
			: await this.getLocalMergeBase(folderManager.repository, pullRequestModel.base.sha, headCommit);
		let localChanges: Change[] | undefined;
		if (startCommit && await this.commitsExistLocally(folderManager.repository, startCommit, headCommit)) {
			localChanges = await folderManager.repository.diffBetween(startCommit, headCommit);

src/test/github/pullRequestModel.test.ts:157

• The remote fallback test stubs getAllFileChangesInfo, so the newly added GitHub implementation is never exercised. The merged pulls.listFiles branch, open-PR compareCommits/merge-base wiring, and parseDiff conversion are therefore unverified—the path used when the local commit range is unavailable. Add focused coverage with mocked GitHub responses for the merged and open-PR cases.

			const getAllFileChangesInfo = sinon.stub(model, 'getAllFileChangesInfo').resolves({
				changes: [new SlimFileChange(mergeBase, '', GitChangeType.RENAME, 'new.ts', 'old.ts')],
				mergeBase,
			});

webviews/components/header.tsx:154

• This condition exposes the action for PRs whose head repository or ref has been deleted. In that case the serialized head is empty and openReadonlyChanges immediately throws for the missing head commit, so the new control is unusable (and the same condition is duplicated in StickyHeader). Hide or disable the action when no head commit is available.

			{!isIssue && isAgentSessionsWorkspace ? <ViewChangesButton /> : null}

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