In principle, any scrolled window should have GTK_SHADOW_IN so the scrollbars
are not above the surface and there is a frame around the scrolled area.

The only exception are the elements of the main window where adding
GTK_SHADOW_IN causes there are too many shadows (or lines in 2D themes)
around the windows and the result isn't nice. So use GTK_SHADOW_NONE
for all main editor scrolled windows. (One additional exception is the
Help->Credits page which is gray and the extra frame doesn't look good.)

Replace frame around VTE with GtkViewport to avoid the extra line around.

Raise the second editor from the splitwindow plugin so it's at the same
level as the main editor.