Widgets missing after startup (was "Toolbar empty after startup")

I'm opening this as a follow-up to #125 (closed).

• Since the switch to gtk3 we have problems with random widgets not showing up after startup.
• This seems to be caused whenever Inkscape changes the size of the window on startup (e.g. to restore the previous window size).
• It can be "fixed" by manually changing the size slightly (widgets appear instantly).

So far it has not been discovered whether this is a bug in Inkscape or an upstream bug (see https://gitlab.gnome.org/GNOME/gtk/issues/2000).

Affected widgets (please add if you find new ones):

• Toolbar / toolbox - #125 (closed)
  A workaround for the toolbar has been merged in !1893 (merged)

• Docking region - #604 (closed)

  • The whole docking region does not appear when opening a new dialog after startup #445 (closed)
  • The docking region can not be closed (empty region remains) when it was shown on startup #445 (closed)
  • The dialogs don't appear at startup (only 1.0.x) #1917 (closed)

• Part of XML and CSS dialogs - #3461

• Layers dialog contents - #3785 (closed)

• Command palette - #2269

  • apparently the shortcut ("?") does not open it
  • it's actually there but only becomes visible after a window resize

• Paint selector - #2373 (closed)
  

• Color palette - #1418

• Likely also causes scroll-bars to bee initially unusable - #1200, #2029