A message sent after the framework has marked a region as focused.
A region may be considered "focused" even when it has lost DOM focus-
this is because the focus may have travelled to an opaque region of
the page (such as an <iframe> element). Additionally, it is
sometimes helpful to be able to switch between multiple tabs (such
as a documentation page and a JupyterLab tab) and still being able
to fire the focus-dependent layout actions, like Ctrl-E, P.
A message sent before a region is about to lose focus.
This occurs when the framework has determined that another region should be focused instead, such as by user action.
A message sent when the framework is about to focus a layout region.
before-focus events are sent immediately before the region is
set to be focused by the framework, and may not line up 1:1 with
related concepts like DOM focus or Phosphor's activate-request.'
Often this event is triggered by a focus DOM event on the region's
subtree. Therefore, regions should not try to #focus() nodes as it
might clobber the user's intended state.
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A message sent after a region has lost focus