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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.

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File types the preview handles

FormatExtension(s)Preview typeEditable?
Markdown.mdRendered markdownYes
Code.py, .js, .ts, .tsx, .jsx, .go, .rs, .java, .c, .cpp, .rb, .php, .sh, .yaml, .json, etc.Syntax-highlighted source with line numbersYes
HTML.html, .htmRendered web pageYes
PDF.pdfPage-by-page rendered PDFNo
PowerPoint.pptxSlide-by-slide previewNo
Excel.xlsx, .xlsm, .csvSpreadsheet with cells and formulasNo
Word.docxDocument preview with formattingNo
Image.png, .jpg, .jpeg, .gif, .svg, .webp, .bmpThe image with metadataNo
Diff—Side-by-side or inline diff viewNo
URL—Rendered web page (inline browser)No

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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.

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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.

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Editing in the preview

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

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.