Currently shellpipe_sync mechanism is essentially a blocking fork and as such freezes up the map UI by design. However, it would be helpful to allow shellpipe_sync scripts to be cancelable from within the UI. E.g. instead of using a blocking fork, shellpipe_sync would operate as a nonblocking fork.
The UI would be overlaid with gray or transparency and there would be a cancel button (and possibly also a counting timer for how long the shellpipe_sync operation has existed for). This would provide transparency for end users that things 'are still working as designed' while running shellpipe scripts and also enable end-users to cancel long-running scripts which don't return in adequate time.
This is relevant for when users put in requests with too large amounts of data (e.g. both for Nominatim/Overpass searches, and additionally for routing/Mobroute requests). The user should be able to 'back-out' and this let's the user more quickly experiment & debug with smaller requests before moving on to larger requests.