This is effectively a port of a topic from 2017 before this was a bug report forum, though the bug has existed (just confirmed it myself) since CS6. Though listed as answered here, the workarounds found are risky and can cause crashes and other unintended results. This needs to be fixed.
Basically, if you have trackers which use enough CPU (super easy to do with large features or search areas, especially on high res footage), the entire UI lags to the point that it cannot recognize keypresses or clicks. Larger trackers will cause it to happen more quickly. It still tracks, and will stop when the clip is over, but in many cases this can leave a clip unstoppably tracking for half an hour with the only workaround being a risky move that could close without saving if the OS is fickle.
versions: all since at least CS6 platform: Windows, possibly others. widespread and well known bug.
BugNeeds More Info
Compositing and VFX
Thank you for reporting this issue and for the link to the older discussion. Unfortunately, we haven't been able to reproduce the AE interface becoming unresponsive during tracking in our testing. Even using 4K footage and large search/feature areas on several minutes of frames, the Stop button for tracking and the space bar both were able to stop the tracking immediately. In seeing how long this bug has persisted, we would certainly like to see it fixed as well.