Somewhat weird TToolbar behavior (D3)

Another one...

I have a regular TToolbar in an application, with buttons drawn from an
imagelist. (Both are standard D3 components.) Now, I have an "open" button
that launches an OPenDialog - and what happens is this:

When clicked (and held...) the button behaves properly, i.e. it is drawn in
a 'depressed' position. But when I release the mouse to trigger the event,
the dialogbox takes a short while to draw, and while it is being drawn, the
'Open' button disappears completely from the toolbar. For a split second
there's a very visible and very ugly empty space on the toolbar in place of
the button... until the button is redrawn in its original state after a
moment. This happens for all buttons that take a noticeable time to execute
(and my "open" routine isn't at all resource-intensive or time-consuming).
This behavior isn't affected by changes to any of the toolbar properties I
can think of, such as 'flat' etc. FWIW, the toolbar doesn't have a 'hot
images' or 'disabled images' assigned.

I'd never seen it happen in any other Win95 application, ever. I actually
tested several apps I have that use regular toolbars, and what always
happens is the the button stays 'down', fully drawn, for as long as it
takes to open the dialog box, after which is returns to normal. Is it a
known D3 quirk?

thanks in advance,

General Frenetics, Discorporated: