This is a regression caused by some previous refactor of the code,
which was preparing it for transition to a tool-based tracking
workflow. Some of the hot zones got tweaked, and a change in the
behavior was done so that when click happens far away from any
slidable marker "widget" all markers gets de-selected (which
matches behavior in 3D viewport).
The pixel tolerance did not apply UI scale factor, and applied
zoom level after squaring (which is wrong calculation of pixel
space in screen space).
Sergey Sharybin