Instead of grid update run a full update, after keeping tab in the background for too long some alert groups are kept in the DOM where they should be purged. Chrome does some really complex background tab throttling, interwebz states that after >=10s in the background a tab can only use 1% of the cpu. Future code might need to use Web Workers API, assuming other vendors will use it too (https://arstechnica.com/information-technology/2017/03/chrome-57-background-tab-suspension-download/). For now let's just reload alerts when user switches to a previously background tab after more than a single refresh cycle, this will ensure that there's no dead alerts presents since that can be confusing
Łukasz Mierzwa