The sidebar can be toggled by F4, View → Sidebar or
a button in the status bar. Hiding the sidebar by dragging the
central splitter to a collapsed state may confuse users if they
can't toggle the sidebar anymore using the normal ways.
This gets even more awkward saving the splitter state in D12489.
This patch simply disables collapsing for the central splitter.
FIXED-IN: 18.08.0