Add investigation notes for browser storage

This collects my notes from investigating the state of browser storage as part
of element-hq/element-web#9109.

docs/storage-notes.md

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+# Browser Storage Notes
+
+## Overview
+
+Browsers examined: Firefox 67, Chrome 75
+
+The examination below applies to the default, non-persistent storage policy.
+
+## Quota Measurement
+
+Browsers appear to enforce and measure the quota in terms of space on disk, not
+data stored, so you may be able to store more data than the simple sum of all
+input data depending on how compressible your data is.
+
+## Quota Limit
+
+Specs and documentation suggest we should consistently receive
+`QuotaExceededError` when we're near space limits, but the reality is a bit
+blurrier.
+
+When we are low on disk space overall or near the group limit / origin quota:
+
+* Chrome
+    * Log database may fail to start with AbortError
+        * Can we use something in memory instead?
+    * IndexedDB fails to start for crypto: AbortError in connect from
+      indexeddb-store-worker
+    * When near the quota, QuotaExceededError is used more consistently
+* Firefox
+    * The first error will be QuotaExceededError
+    * Future write attempts will fail with various errors when space is low,
+      including nonsense like "InvalidStateError: A mutation operation was
+      attempted on a database that did not allow mutations."
+    * Once you start getting errors, the DB is effectively wedged in read-only
+      mode
+    * Can revive access if you reopen the DB
+
+## Cache Eviction
+
+While the Storage Standard says all storage for an origin group should be
+limited by a single quota, in practice, browsers appear to handle `localStorage`
+separately from the others, so it has a separate quota limit and isn't evicted
+when low on space.
+
+* Chrome, Firefox
+    * IndexedDB for origin deleted
+    * Local Storage remains in place
+
+## Persistent Storage
+
+Storage Standard offers a `navigator.storage.persist` API that can be used to
+request persistent storage that won't be deleted by the browser because of low
+space.
+
+* Chrome
+    * Chrome 75 seems to grant this without any prompt based on [interaction
+      criteria](https://developers.google.com/web/updates/2016/06/persistent-storage)
+* Firefox
+    * Firefox 67 shows a prompt to grant
+    * Reverting persistent seems to require revoking permission _and_ clearing
+      site data
+
+## Storage Estimation
+
+Storage Standard offers a `navigator.storage.estimate` API to get some clue of
+how much space remains.
+
+* Chrome, Firefox
+    * Can run this at any time to request an estimate of space remaining
+* Firefox
+    * Returns `0` for `usage` if a site is persisted
+
+## Actions
+
+* Add console logging to record storage state
+* Add debug panel in Settings to investigate storage state
+* Catch case of IndexedDB being deleted and alert user to sign out
+* Consider storage estimation to predict issue coming up
+* Consider switching to persistent storage to avoid this issue
+* Investigate persistent storage for Electron where we'd like it to default on
+* Move user's own Olm keys to localStorage, so that they can recover when
+  IndexedDB disappears [notes](https://matrix.to/#/!RbKRuLTvHIhArQcvjr:matrix.org/$1553091062267114tiefP:matrix.org?via=matrix.org&via=lant.uk&via=t2l.io)