This Obsidian plugin keeps files opened until explicitly closed, turning the app into a more "proper" multi-document editor. This means that when switching from document A to document B, and then back to document A, you still have document A's state: your undo/redo history, your position in the file, your selection, and so on.
Some important notes:
Behind the scenes, the documents are closed. The plugin saves the undo/redo history, last selection, and more, before the underlying editor is discarded. This means that there's very little impact on memory usage.
This plugin is using a number of horrible hacks to achieve its goal, and should be considered a "proof of concept" more than anything else. It could be broken with any Obsidian update.