[Dave Rayment (daverayment)]

Summary of the Pull Request

Previously, only one file could be previewed via the CLI. This adds support for multiple files, adds the ability to preview folders, and fixes a number of related issues with the CLI parameter parsing and validation code.

PR Checklist

• Closes: [Peek] The Runner PID is not validated, resulting in a zombie process #49667
• Closes: [Peek] A numeric final argument in CLI mode is assumed to be the Runner PID #49666
• Closes: [Peek] Opening a folder in CLI mode miscategorises it and shows 0 byte size #49665
• Closes: [Peek] Opening a file via the CLI errors if the path is relative #49664
• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

1. Relative paths were not expanded into full paths

Submitting relative paths via the CLI passed file validation (the file was recognised as existing), but when passed to Shell APIs for opening, a full path is required, resulting in Peek showing an error message. This is fixed in the new InitializeFromCliPaths() method which validates the path arguments and expands out the valid ones into full paths.

2. Only a single file was supported

The previous CLI code

_selectedItem = new SelectedItemByPath(filePath);

meant that only 1 file was ever supported. Multiple files and/or folders were not.

To support multiple files via the command line this PR:

1. Introduces a SelectedItemsByPaths class (notice the plurals), which inherits from SelectedItem and returns true from Matches() if the passed in path matches any of the paths passed in via the command line.
2. Updates the Items field in the MainWindowViewModel from NeighboringItems to a more generic IReadOnlyList<IFileSystemItem>. NeighboringItems is closely tied to the Explorer Shell API query path and should not have been exposed directly to the model and front-end.
3. Adds a HasMultipleItems property, replacing the NeighboringItemsQuery.IsMultipleFilesActivation, which controls whether the title bar shows the number of items and the current item number. This is similar to 2., where NeighboringItems and related types should not be part of the front-end.

Screenshot showing current item number and total items in title bar:

Folders are supported by this code in the new InitializeFromCliPaths(), which creates FileItem or FolderItem instances:

        foreach (var path in paths)
        {
            string name = Path.GetFileName(path);
            items.Add(Directory.Exists(path)
                ? new FolderItem(path, name, path)
                : new FileItem(path, name));
        }

Folder view from running PowerToys.Peek.UI.exe . (i.e. current folder):

3. Startup and validation fixes and increased robustness

1. Added --runner-pid as a required parameter when supplying the Runner's process ID
   This fixes the issue where the last argument would always be interpreted as the Runner's process ID if it was numeric. With the addition of folder previews via the CLI, this was necessary, otherwise folders with numeric names could not be previewed if they were listed as the last argument.

2. Validated Runner PID
   The numeric PID argument was always assumed to be correct previously. Providing an incorrect value manually would result in Peek opening, but it would not be linked to the Runner and could only be closed via a Task Manager kill. This PR validates that the PID is a running process and the linked exe is called "PowerToys.exe".

3. Refactored startup code
   A new LaunchArgumentsClassifier class determines whether Peek is being opened via the Runner or the CLI, with new TryHandleRunnerLaunch() and TryHandleCliLaunch() methods validating and preparing the items for navigation.

4. Other changes

The more generic navigation of IFileSystemItems means that the CLI does not have to be treated as a special case in the MainWindowViewModel, so _isFromCli has been removed.

Validation Steps Performed

1. New unit tests for the LaunchArgumentsClassifier code
2. All new and existing unit tests pass:

3. Manual tests:

• Open Peek via Explorer selection of 1 file, 1 folder, and multiple files and folder(s) mix
• Open Peek via CLI with 1 file, 1 folder, and multiple files and folder(s) mix
• Test navigation for all above combinations
• Confirm Peek process closes on CLI validation error
• Confirm Peek process closes when opened via the CLI when user exits the application normally
• Confirm file delete still works, independent of how Peek was opened

[Copilot]

🟡 Changes recommended

Folder display names and item-count visibility have unresolved edge-case defects, and the core CLI navigation path lacks automated coverage.

3 open findings

• HasMultipleItems remains true after item deletion · New
• Folder previews have blank names for trailing separators and roots · New
• Add CLI navigation coverage for mixed file and folder arguments · New

What changed in this PR

Adds robust Peek CLI support for multiple files and folders while distinguishing CLI launches from Runner launches.

Changes:

• Adds explicit Runner PID parsing and validation.
• Resolves CLI paths and supports mixed multi-item navigation.
• Generalizes item-count UI state and adds argument-classifier tests.

File	Description
src/​modules/​peek/​peek/​dllmain.cpp	Passes the Runner PID using an explicit flag.
src/​modules/​peek/​Peek.UI/​PeekXAML/​MainWindow.xaml	Binds title visibility to generalized item state.
src/​modules/​peek/​Peek.UI/​PeekXAML/​App.xaml.cs	Classifies launches, validates PIDs, and resolves CLI paths.
src/​modules/​peek/​Peek.UI/​NativeMethods.txt	Normalizes the final entry formatting.
src/​modules/​peek/​Peek.UI/​Models/​SelectedItemsByPaths.cs	Represents multi-path CLI selections.
src/​modules/​peek/​Peek.UI/​MainWindowViewModel.cs	Creates folder/file items and enables CLI navigation.
src/​modules/​peek/​Peek.Common/​Helpers/​LaunchArgumentsClassifier.cs	Separates Runner and CLI argument forms.
src/​modules/​peek/​Peek.Common.UnitTests/​LaunchArgumentsClassifierTests.cs	Tests launch argument classification.

🧠 Review effort: Balanced

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