Pretty cool! Would you be able to add some other features though like hot key remappings? Would like to be able to rewind, exist, create and load save states from gamepad and keyboard. Also an easier way to exit while in fullscreen mode especially from keyboard instead of doing alt + f4.
You should always be able move your mouse to top of the screen to get the menu bar. If you set the tool to start in fullscreen you can also use and communicate F12 to switch between fullscreen and window. :-)
NESbag should not be an alternative to a full emulator therefore I didn't implemented a savegame or rewind function.
A save/load ability will never be part of it because a NES game should work exactly how it work. External saves are cheating. ;-)
Like to keep simple, for what it is.
The last few points are fair but I would at least like to be able to quickly exit the emulator by either just hitting esc on keyboard or a certain other button on a gamepad like a shoulder button for example or some short button combo if those would be possible to implement. Could be hotkeys you can set like I previously mentioned.
Much easier than having to get out of full screen before being able to quit.