FIX: attempts to improve screen-tracker logic (PR #8113)

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Un-optimized code, mostly for experiment, will improve it if it proves useful.

The title of this pull request changed from “EXPERIMENTAL: stops/starts screen-tracking on tab focus” to "@jjaffeux FIX: attempts to improve screen-tracker logic

The title of this pull request changed from “@jjaffeux FIX: attempts to improve screen-tracker logic” to "FIX: attempts to improve screen-tracker logic