Cannot play multiple notes with virtual keyboard

[pgregory]

Description

Currently whenever a key is released, the playing note is stopped, even if another note has been triggered since, making it difficult to play notes effectively.

Steps to reproduce

1. Load a song with some samples.
2. Press "z" key to play an instrument, hold it.
3. Press "x" key to play another, while still holding the "z", and also hold the "x" key.
4. Release the "z" key, the sound stops, even though "x" is still held.

Versions

• OS: macOS Sierra
• Browser: Chrome