Pencil is an open-source GUI prototyping tool for creating wireframes and mockups across platforms. It is built as an Electron desktop application with a custom UI framework that includes a canvas for SVG-based shape editing, a stencil/collection system for reusable UI components, and export pipelines to SVG, PDF, ODT, HTML, and PNG. The architecture is a monolith with a main process (app/index.js) managing windows and IPC, and a renderer process handling the UI and canvas interactions. The codebase shows a migration from legacy Mozilla XULRunner to Electron, resulting in a mix of old XUL/XPCOM code and modern JavaScript/Node.js patterns. Key data flows involve loading shape definitions from XML collections, manipulating shapes on an SVG canvas, and serializing documents to custom archive formats (.ep, .epz, .epgz). External dependencies include Electron, various npm packages (adm-zip, archiver, jimp, perfect-freehand), and system fonts. The repository also contains a web-based asset management subsystem (DataTable, MapView) that appears to be a separate concern embedded within the same codebase.