#1285 Render artifacts make app visually unusable

[Benjamin Johnson]

 Versions and environment
    FreeMind version: 1.0.1
    Operating system - name and version. An example: "Windows 10 Pro".
    Java version: Version 8 update 201, also occured on 1.7 (I don't have a record of which update # though)
Bug description
    While creating a mindmap, ocassionally after adding new child nodes, any time a control draw is triggered, it is rendered over a copy of the whole window. The app continues to work fine, but the graphical artifacts make it pretty much unusable. This happens consistently and often (typically after about 10 minutes of adding and editing child nodes), saving and restart work, but this bug makes the app very annoying to use.