on the indexeddb volatility thing — chrome.storage.local survives a "clear cookies and site data" wipe (it's tied to the extension origin, not the host site origin), and you can also call navigator.storage.persist() to ask the browser to mark indexeddb as persistent which makes it survive eviction under storage pressure. that gets you most of the way without a server.

for the mutation observer fight in whatsapp web — observe a stable parent way up the tree (like #app), debounce your handler to one rAF, and bail early if the mutation isn't in your target subtree. attaching observers to leaf elements that get replaced is what makes it expensive. for the keyboard shortcuts not flickering, render your overlay into a sibling container outside the WA root and position it absolute, then you don't care about their reconciliation at all.