Preview panel

The Preview panel is Headmaster's multi-format file viewer. When the agent writes a file or you click a file in the File Manager, the preview panel opens next to the chat showing the file's contents rendered in the appropriate format.

File types the preview handles

| Format | Extension(s) | Preview type | Editable? | |---|---|---|---| | Markdown | .md | Rendered markdown | Yes | | Code | .py, .js, .ts, .tsx, .jsx, .go, .rs, .java, .c, .cpp, .rb, .php, .sh, .yaml, .json, etc. | Syntax-highlighted source with line numbers | Yes | | HTML | .html, .htm | Rendered web page | Yes | | PDF | .pdf | Page-by-page rendered PDF | No | | PowerPoint | .pptx | Slide-by-slide preview | No | | Excel | .xlsx, .xlsm, .csv | Spreadsheet with cells and formulas | No | | Word | .docx | Document preview with formatting | No | | Image | .png, .jpg, .jpeg, .gif, .svg, .webp, .bmp | The image with metadata | No | | Diff | — | Side-by-side or inline diff view | No | | URL | — | Rendered web page (inline browser) | No |

Preview panel with multiple tabs

Screenshot placeholder: The preview panel showing multiple file tabs and a markdown preview.

Panel layout

The Preview panel is a tabbed interface:

Multiple tabs — open multiple files at once and switch between them. Each tab shows the file name and a close button.
Split view — drag a tab to split the panel into two side-by-side views. Useful for comparing two files.
Pop out — click the pop-out button to open the preview in a separate window. Useful for putting a preview on a second monitor.

The panel can be:

Docked right (default) — preview on the right, chat on the left.
Docked bottom — preview on the bottom, chat on top.
Hidden — click the toggle to hide the panel and give the chat full width.

Automatic previews

The preview panel opens automatically when:

The agent writes a new file — the preview pops up showing the result.
The agent creates a new Office document — the preview shows the document.
The agent edits an existing file — the preview shows the diff.

This is the default behavior. You can disable auto-open in Settings → My Headmaster → Look → Auto-open previews.

Editing in the preview

For Markdown, code, and HTML, the preview panel is also an editor:

Markdown editor

Edit tab — write markdown in a text editor with syntax highlighting.
Preview tab — see the rendered markdown in real time.
Switch between tabs to edit and preview.
Save with Cmd/Ctrl + S — writes the file to disk.

Code editor

Full Monaco-based code editor (the same engine that powers VS Code).
Syntax highlighting for 50+ languages.
Line numbers, code folding, multi-cursor, find and replace.
Save with Cmd/Ctrl + S.

HTML editor

Edit tab — write HTML/CSS/JS in a code editor.
Preview tab — see the rendered web page in real time.
Save with Cmd/Ctrl + S.

Diff viewer

When the agent edits an existing file, the preview panel shows a diff:

Side-by-side — original on the left, modified on the right. Added lines in green, removed lines in red.
Inline — changes shown inline in a single view. Added lines prefixed with +, removed with -.

Toggle between side-by-side and inline with the toolbar button.

The diff viewer supports:

Syntax highlighting in both panes.
Word-level diffs (not just line-level).
Minimap for navigating large diffs.

Preview history

The Preview panel keeps a history of recently viewed files. Click the history dropdown in the toolbar to see the last 20 files you've previewed. Click any entry to re-open it.

Context menu

Right-click in the preview panel for context menu options:

Copy — copy the selected text or content.
Save — save changes (for editable formats).
Save as — save a copy with a new name.
Reveal in folder — open the file's location in your OS file manager.
Close tab — close this preview tab.
Close all tabs — close all open preview tabs.
Open externally — open the file in its default OS application (e.g., open a .pptx in PowerPoint).

Office document preview

For Office documents (.pptx, .xlsx, .docx), the preview panel shows a read-only rendered view:

PowerPoint — slide navigator on the left, current slide on the right. Click a slide to jump to it.
Excel — sheet tabs at the bottom, cell grid in the main area. Formulas are shown as values (calculated).
Word — rendered document with headings, paragraphs, tables, and images.

Office previews are read-only — to edit, click Open externally to open in PowerPoint, Excel, or Word.

PDF preview

The PDF viewer renders pages one at a time with:

Page navigator (jump to any page).
Zoom in / out.
Fit to width / fit to page.
Text selection (copy text from the PDF).
Search within the PDF.

PDFs are read-only in the preview panel.