Sessions: Add #file context completions to new-chat input

[osortega]

Fixes: #299057
Adds # completions to the sessions NewChatViewPane input editor, scoped to the workspace selected in the dropdown picker.

Changes

New file: variableCompletions.ts

• VariableCompletionHandler registers a completion provider on the sessions-chat URI scheme (same pattern as SlashCommandHandler)
• Typing # immediately shows files from the selected workspace
• For file:// and vscodeRemote schemes: uses ISearchService.fileSearch()
• For virtual schemes (e.g. github-remote-file://): walks the file tree via IFileService.resolve()
• Recent workspace-scoped files from editor history (IHistoryService) are always shown
• Accepted completions insert #file:<name> text with slash-command-style decorations (colored background/foreground via chatSlashCommandForeground/chatSlashCommandBackground, rounded corners)
• Files are also added as attachment pills via NewChatContextAttachments

Modified: newChatContextAttachments.ts

• Added public addAttachments() method (wraps private _addAttachments())

Modified: newChatViewPane.ts

• Imported and instantiated VariableCompletionHandler alongside SlashCommandHandler
• Passes the editor, context attachments, and workspace URI getter

Design decisions

• No @ completions — only # for file context, matching what makes sense for the sessions pre-chat state
• No #sym:, #selection, #session — symbols require open editors/language servers (unavailable pre-session), selection/session don't apply in the sessions app
• Workspace-scoped — all results filtered to the workspace selected in the dropdown, matching the "Add Context..." button behavior
• No core dependencies added — all code lives in vs/sessions/, imports only from allowed layers

[Copilot]

Pull request overview

Adds #-triggered file-context completions to the Sessions “new chat” input (scheme sessions-chat), scoped to the workspace chosen in the picker, and wires accepted items into context attachment pills.

Changes:

• Introduces VariableCompletionHandler to provide #file: suggestions from editor history + workspace search / filesystem walk, and decorates inserted #file: tokens.
• Exposes NewChatContextAttachments.addAttachments() for programmatic addition of context pills.
• Instantiates the new completion handler in NewChatViewPane alongside the existing slash command handler.

Show a summary per file

File	Description
src/vs/sessions/contrib/chat/browser/variableCompletions.ts	New # completion provider + decorations + command to add selected resources as attachments.
src/vs/sessions/contrib/chat/browser/newChatContextAttachments.ts	Adds a public wrapper to add attachments from completion acceptance.
src/vs/sessions/contrib/chat/browser/newChatViewPane.ts	Wires VariableCompletionHandler into the new-chat editor lifecycle.

Copilot's findings

Comments suppressed due to low confidence (2)

src/vs/sessions/contrib/chat/browser/variableCompletions.ts:238

• _addFileAndFolderEntries always calls into _addEntriesViaSearch / _addEntriesViaFileService, even when the user has only typed # and pattern is empty. That means ISearchService.fileSearch can run a workspace-wide search with an empty filePattern, which can be expensive on large repos. Consider gating the search/tree-walk behind a non-empty pattern (and relying on the history suggestions for the initial # list), similar to how workbench chat variable completions only search when pattern is truthy.

		// SEARCH — always run to populate initial results (empty pattern returns scored files)
		if (workspaceUri.scheme === Schemas.file || workspaceUri.scheme === Schemas.vscodeRemote) {
			await this._addEntriesViaSearch(workspaceUri, pattern, seen, makeItem, result, token);
		} else {
			await this._addEntriesViaFileService(workspaceUri, pattern, seen, makeItem, result, token);
		}

src/vs/sessions/contrib/chat/browser/variableCompletions.ts:272

• For file:// and vscodeRemote workspaces, _addEntriesViaSearch only adds FileKind.FILE suggestions, so directories can never be suggested/attached via # completions. For non-searchable schemes you do add directories via the file-service walk, so behavior differs by scheme even though NewChatContextAttachments supports kind: 'directory'. Either add directory suggestions for local/remote too (e.g. via a lightweight fileService resolve for top levels / limited depth) or update the UX/docs to clarify that # completions only attach files on searchable workspaces.

	/**
	 * Uses the search service to find files/folders — works for `file://` and `vscodeRemote` schemes.
	 */
	private async _addEntriesViaSearch(
		workspaceUri: URI,
		pattern: string | undefined,
		seen: ResourceSet,
		makeItem: (resource: URI, kind: FileKind, description?: string, boostPriority?: boolean) => CompletionItem,
		result: CompletionList,
		token: CancellationToken,
	): Promise<void> {
		const excludePattern = getExcludes(this.configurationService.getValue<ISearchConfiguration>({ resource: workspaceUri }));

		try {
			const searchResult = await this.searchService.fileSearch({
				folderQueries: [{
					folder: workspaceUri,
					disregardIgnoreFiles: false,
				}],
				type: QueryType.File,
				filePattern: pattern || '',
				excludePattern,
				sortByScore: true,
				maxResults: 100,
			}, token);

			for (const match of searchResult.results) {
				if (!seen.has(match.resource)) {
					seen.add(match.resource);
					result.suggestions.push(makeItem(match.resource, FileKind.FILE));
				}
			}

• Files reviewed: 3/3 changed files
• Comments generated: 2