mini-player size is changed when I unplug from dual screen

[marquesarthur]

OS: MacOS 10.13.2
setup: MacOS + Widescreen monitor connected through a thunderbold VGA adapter

Steps to reproduce:

1. Open the player
2. Turn the mini-player on
3. Move the window to a second display, connected through a thunderbold VGA adapter
4. Disconnect the adapter

Result: size of the mini-player consumes almost the entire screen, which is at least awkward for a mini player.

Attached follows a print:

[marquesarthur]

Yet another print with the entire desktop screen:

Also, pressing the switch to full player button and then back to mini-player doesn't fix the problem and now I have a huge size mini-player :(

[marquesarthur]

@jostrander @MarshallOfSound Can I get instructions on how to launch the mini player in a development environment? I would be glad to help if someone give me instructions about that.
The ones on the README.md are a little bit blurry and I could also update them for others trying to contribute with the project.