Bug 864833

Description of problem:
If I want move the boundary event along the boundary of the task then sometimes happened that the boundary event is changed to the intermediate catch event. 

Version-Release number of selected component (if applicable):
BRMS 5.3.1 ER1, designer 2.3.1-DEV

How reproducible:
Depends on position of mouse (if mouse is outside of the task then boundary event is changed to intermediate catch event).
The green boundary of task appears when the mouse is over the task: 
 - If I catch event at the top then the green boundary show up, the mouse is over the task and the boundary event stays. 
 - If I catch event at the bottom then the green boundary does not show up, the mouse is out of the task and the boundary events is changed. 


Steps to Reproduce:
1. create new BPMN2 asset
2. create the process (Start->Task->End)
3. attached boundary event
4. catch event at the bottom (mouse is out of the task) and move along a boundary of the task.
  
Actual results:
The boundary event is changed to intermediate catch event. It depends on position of mouse.

Expected results:
The boundary event stays. It depends on position of node (corners or midpoint). Please if you choose midpoint then add some position tolerance.

Additional info:
You can find this problem only if you look inside BPMN2 source code or you use validator (Mark by alert "Node has no incoming connections.". But validator is broken for this issue, now. See bz 864794). There is no difference between property bars (same title of property bar).