Playground: formatting stops editting

[aspiwack]

Describe the bug
In the playground, when the formatter is running (and “formatting...” is displayed in the Output frame), any edits to the Input frames is not displayed (what you type is taken into account, but only when formatting is over).

This is rather problematic when on-the-fly formatting is turned on, and the interface can decide at any time to prevent you from properly editing your buffer.

Expected behavior
The Input buffer should update in real time even while a formatting process is running.

[aspiwack]

In fact, while formatting is going on, the entire interface is unresponsive, which is quite frustrating.