Viewport
The Obsidian editor supports huge documents with millions of lines. One of the reasons why this is possible, is because the editor only renders what's visible (and a little bit more).
Imagine that you want to edit a document that is too big to fit on your monitor. The Obsidian editor creates a "window" that moves across the document, only rendering the content within the window (and ignoring what's outside). This window is known as the editor's viewport.
Whenever the user scrolls through the document, or when the document itself changes, the viewport becomes out-of-date and needs to be recomputed.
If you want to build an editor extension that depends on the viewport, refer to View plugins.
This page aims to distill the official CodeMirror 6 documentation for Obsidian plugin developers. For more information on state management, refer to Viewport.