Main window

The main window is the top-level container. It switches between three pages via an internal QStackedWidget:

1. A centered placeholder when no session is loaded.
2. Level 1 — timeline overview (see §5.2).
3. Level 2 — detail pair view (see §5.3).

The window opens at 1400 × 800 by default and is fully resizable.

Title bar

Shows one of three states:

Title	Meaning
MIDI-Camera Alignment Tool	No participant loaded
MIDI-Camera Alignment Tool — Participant 042	Session loaded, no unsaved changes
MIDI-Camera Alignment Tool — Participant 042 *	Session loaded, unsaved changes

A trailing * appears whenever _dirty is set: after applying a global shift, adding/deleting an anchor, toggling an anchor, or rebasing paths during a load. The * disappears only after a successful Save Alignment.

Status bar

Below the main pages, the status bar shows one of:

• No participant loaded — initial state.
• Participant 042 | 12 MIDI files | 12 camera clips | Global shift: 0.000s | Anchors: 5 — full summary, updated after every state change.
• Level 2: trial_001.mid + C0001.MP4 — while a Level 2 pair is active.
• Saved: /path/to/alignment.json — transient confirmation after Save Alignment.

File menu

Single top-level menu — no toolbar, no other menus.

Item	Shortcut	Behaviour
Open Participant…	Ctrl+O	Folder picker. Prompts to save first if dirty. On selection, runs ParticipantLoader.load and swaps state to Level 1. Shows a post-load warning dialog if any files were skipped. If no .mid or .MP4 files were found, shows a "No Files Found" warning and does not change state.
Save Alignment…	Ctrl+S	File picker for *.json. Writes the session atomically via tempfile + os.replace. Disabled until a session is loaded. After success, clears the dirty flag, drops the * from the title, and shows the file path in the status bar.
Load Alignment…	Ctrl+L	File picker for *.json. Prompts to save first if dirty. Rebases paths if the stored participant_folder doesn't exist on disk (see below).
Exit	Ctrl+Q	Closes the window. Prompts to save first if dirty. Releases the Level 2 video and intensity workers.

Unsaved-changes prompt

When closing, opening a new participant, or loading a JSON while _dirty is true, a modal QMessageBox appears:

You have unsaved changes. What would you like to do?

Save | Discard | Cancel

• Save — opens the Save dialog. If save succeeds, proceeds with the original action. If save fails or is cancelled, stays on the current session.
• Discard — proceeds with the original action, dropping all unsaved changes.
• Cancel — aborts; stays on the current session.

Moving participant folders

When you Load Alignment… and the JSON's participant_folder no longer exists on disk, a folder-picker dialog appears with this header:

Participant folder not found: {old path}

Select the new location:

If you pick a new folder, persistence.rebase_paths remaps every file_path, mp4_path, and xml_path under that new base and the session is marked dirty (so you're nudged to re-save the JSON with the corrected paths). If you cancel the folder picker, the load aborts and the previous session remains active.

Error dialogs

All errors come through a single router (_show_exception in alignment_tool/ui/main_window.py:260-266):

• MediaLoadError subclasses → QMessageBox.critical titled with the exception class name.
• PersistenceError subclasses → QMessageBox.critical titled with the exception class name.
• InvariantError subclasses → QMessageBox.warning (less severe).
• Any other exception → QMessageBox.critical(title="Error") with the exception string.

See §7.2 Error messages for an exhaustive list of what each exception means.