Mailto Inside от Guy Louzon

Mailto Inside intercepts mailto: links on any page and replaces the browser's native handler with a small in-page compose form. When you hit send, it opens a pre-filled Gmail draft in a new tab — no OAuth, no API keys, and nothing is ever transmitted by the extension itself.

Clicking a mailto: link on a page that isn't your default mail client usually triggers an "open with…" dialog, or silently does nothing. Mailto Inside intercepts the click, shows a lightweight compose form right on the page, and hands the finished message to Gmail's own web compose URL — so you finish and send from an already-authenticated Gmail tab.

Source code here:
https://github.com/guylouzon/mailto-inside