v4: Steps not working in separate window

[voliva]

When upgrading to v4 I noticed that the steps are not synchronised between windows: when pressing arrow keys in the presenter window, only the presenter window has the steps changing while the external window is kept frozen (and viceversa)

As an example, the file in examples/steps/deck.mdx reproduces this issue.

The workaround is to focus on the external window to move forward/backward, although the presenter window won't update.

Chrome: 80 Firefox: 74 MacOS