Canvas and message windows shown at confusing times in blocks

There's room for a lot of improvement in when various UI elements are shown. I'd propose:

a) The sidebar (including the source code box) is always visible, as it is now.
b) The message box starts out hidden, and is shown from the moment the user clicks Run, until they click Stop.
c) The canvas is show when there is a running program, and there are no errors.

Currently, the message window is always visible even with no program running, and the canvas starts out visible (but is hidden by the Stop button). These seem wrong.

Trace the block UI state handling and identify where the sidebar, message box, canvas, Run, and Stop controls are rendered or updated. Compare the current behavior with the three requested visibility rules, then verify that the message box and canvas appear only during the specified running and error-free states.

Written by the indexing model from the issue text.