PCLVisualizer slows down considerably when calling spinOnce from keyboardCallback

Your Environment

• Operating System and version: Ubuntu 18.04
• Compiler: gcc 7.4.0
• PCL Version: 1.9
• VTK Version : 6.3

Context

I have created a simple cloud viewer which loops through every .pcd file in a directory and
upon hitting the "n" key, updates the currently viewer point cloud on screen

When i call the viewer->spinOnce() method from inside the callback function, after a few
callbacks the viewer slows down to the point where it's unusable and i have to interrupt the program

Solution, simply remove the viewer->spinOnce() method from inside the keyboard callback and keep it only in the "main loop"

Code to Reproduce

• slow_viewer.cpp contains the source code for the "project"
  slow_viewer.zip

I have no idea if this is a bug.
I could not find an implementation of the spinOnce method to investigate.

I am aware of #2386 but upon looking at the memory usage of the programm (with htop) it does not seem to leak memory..

I guess this is more of a question of why is this happening, thnx :)

Start with the attached slow_viewer.cpp and reproduce the slowdown by calling PCLVisualizer::spinOnce from the keyboard callback, comparing it with calling spinOnce only in the main loop. Trace the available spinOnce implementation and callback behavior; done means identifying whether this is a bug and documenting or addressing the cause.