multiple item selection reacts wrong to left mouse button up/down in explorer filebrower

ros trunk r75637 gcc dbg:

When selecting multiple files while holding ctrl the system should change the selection on LBUTTONUP, but it erroneously changes the multi-selection already on LBUTTONDOWN.

This leads to very unintuitive follow-up errors like the following sequence:
select 3 files with left mouse button (while holding ctrl)
make sure no mouse button is pressed anymore and ctrl is released
now press-n-hold ctrl again (with the aim to enforce copy in next step)
when trying to start the drag-operation by left-mouse-button-click-n-hold we will erroneously deselect the file where we "grabbed the multi-selection"

(This does not happen when you just start to drag (without holding ctrl).)

If we'd change multi-selections properly on LBUTTONUP instead LBUTTONDOWN (like W2K3 does), there wouldn't be any problem.