Collapsed markdown header doesn't expand when creating cell under it if cell is not active

[roblourens]

Related to #120641

Apr-29-2021.21-25-41.mp4

When I create a cell in an editor that isn't the active group, the markdown header parent is only expanded in the active group, not the one I clicked in.

[rebornix]

It's a mix of two features:

• click on insert toolbar will not steal focus
• changes to no focused editor will not trigger relayout if possible (in this case, the left side editor only gets a model change, so no folding/collapse change

[rebornix]

The right behavior will be:

• Clicking on the insert toolbar will focus the editor containing that toolbar
• New cell inserted in the editor and folding region expaned
• The other editor will not update