I've had another look at this and for some reason I cannot reproduce the 'External SIGEGV' error in Windows10. However when I left click the control on the right side the trackbar, the indicator jumps to the left. After some experiments, if I place the....
inherited MouseDown(Button, Shift, X, Y);
line at the very end of the MouseDown procedure, rather than at the beginning, then all mouse events seem to give the correct behaviour in both Linux and Windows10.
similar message added to bug report