Themes can nowadays set padding on GtkLabel widgets (e.g. Ambiance and
Radiance do this for labels in primary toolbar buttons). This breaks our
hack to force a size request for the pathbar labels, since we measure
the PangoLayout directly instead of measuring the GtkLabel it's part of
(which includes the border/padding values from the theme).

Fix this by measuring the size requisition of GtkLabel directly; for
this to work effectively, we need to pack an (invisible) additional
label in the button GtkBox, always set the text to both labels and
update the requisition of the non-bold one to
MIN(MAX_POSSIBLE_WIDTH, MAX (width, bold_width)) every time a
size-request cycle is called.

https://bugzilla.gnome.org/show_bug.cgi?id=678341