Well, while packing my project in order to send it to you, I found my problem.
I added a call to newFrame() (method from the demo) in my main loop, and now it works. So, I don't really understand how the demo can work without that, I guess I need time to learn ImGui.
Anyway, thank you for your help Elect, and thank you for your jvm binding.
Yeah I saw that the newFrame() is called by the buildCommandBuffer(), but I didn't though the commandBuffer would be rebuilt every frame.
For the record : the demo do that to update the scisors and eventually resize the buffers. I think a better approach would be to rebuilt it only when something is moved/resized on the UI.