Mouse drag does not work in the left half of window

[dims12]

I wrote the following project: https://github.com/dims12/GMapFXTest01

And I see, that after initial run, I can drag map only if started in the right half of window. If drag for the left half, drag is ignored.

Proof: https://www.youtube.com/watch?v=nzxP5P6SnSk

Why?