Previously, when a cache was invalidated and every tab/iframe issued a getStyles request, we previous needlessly accessed IndexedDB for each of these requests. It happened because 1) the global cachedStyles was created only at the end of the async DB-reading, 2) and each style record is retrieved asynchronously so the single threaded JS engine interleaved all these operations. It could easily span a few seconds when many tabs are open and you have like 100 styles.
Now, in getStyles: all requests issued while cachedStyles is being populated are queued and invoked at the end.
Now, in filterStyles: all requests are cached using the request's options combined in a string as a key. It also helps on each navigation because we monitor page loading process at different stages: before, when committed, history traversal, requesting applicable styles by a content script. Icon badge update also may issue a copy of the just issued request by one of the navigation listeners.
Now, the caches are invalidated smartly: style add/update/delete/toggle only purges filtering cache, and modifies style cache in-place without re-reading the entire IndexedDB.
Now, code:false mode for manage page that only needs style meta. It reduces the transferred message size 10-100 times thus reducing the overhead caused by to internal JSON-fication in the extensions API.
Also fast&direct getStylesSafe for own pages; code cosmetics
The injection code also runs outside of onInstalled event so we check first if a content script belonging to our execution context "generation" is already injected. This can happen on browser startup: the background page is loaded in several seconds after the normal web page tabs are loaded with our content script(s) already injected. The check itself is simply a "ping" message to each tab that should return true if the content script is alive and kicking.
