treeview selection+drag model is not HIG compliant

[Paul Davis <paul linuxaudiosystems com>]

take a treeview with some displayed rows. click on a row. it is
selected. shift-click on some other row. now several rows are selected
(assuming MULTIPLE was chosen). now press but do not release the mouse
on one of the selected rows. the selection changes to just that row.

this doesn't follow the HIG, because the treeview is changing the
selection before any possible drag is initiated. clicking on an already
selected row should not have any consequence until button release.

the only excuse i can see for this is the incredibly disappointing fact
that the treeview only supports single row drag-n-drop natively,
therefore the selection has to necessarily collapse to a single row
before the drag begins. however, in gtkmm it has proven quite simple to
derive a class from TreeView that supports multi-row DnD, apart from
this problem with multi-select-then-press-n-drag collapsing the
selection to a single row. multi-select-n-drag as a single mouse
operation works just fine.

if someone can confirm that this is not "intended" behaviour, i will
file it in bugzilla.

--p