The toolbar should support a vertical offset from the top of the web page #60

Focus then scroll

1. Focus the editor.
2. Scroll the web page until the toolbar reaches the green box boundaries.

Expected:

1. The toolbar should stick to the green box instead of the viewport edge, respecting toolbar.viewportTopOffset configuration.
2. At some point, the toolbar should disappear below the green box, leaving some space for the last paragraph of the content to remain visible.

Scroll then focus

1. Scroll the web page until the toolbar is underneath the green box.
2. Focus the editor.

Expected:

1. The toolbar should stick to the lower boundary of the green box.