For some reason the cursor of X is out of sync with the cursor of VNC.
For example, it reaches to 5cm top and then exit the region as if it was at 0cm top.
Workaround is to go with cursor to the lowest/rightest location, then it re-sync until out of sync again.
Oh... did not set me as submitter...
This is a known problem. On bug tracker, you can find a few records.
For yet I have not got time to fix it.