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.