FileMaker windows don't redraw when a plug-in shows a modal dialog
Windows XP Home (regularly updated, all service packs)
In the Windows version a plug-in displays a modal dialog using one of DialogBox() family of functions. The dialog displays, but when I move it, the underlying windows don't redraw. The standard elements (window title, border, scrollbars, toolbar, etc.) do redraw, but the rest of the client area does not, so the dialog leaves a trail.
I've met this with my own plug-in, which is currently in development; somehow other plug-in authors seem to work around this issue (I'm trying to contact these developers, but I haven't received a definite answer yet.) But I noticed the similar behavior with Tony Kargas's Dialog plug-in (http://kargas.net/dialog.html). Open the sample file, go to the last tab that shows standard dialogs and try to display the Select Folder dialog. On my system it erases, but doesn't redraw the underlying FileMaker window. This doesn't happen every time, sometimes it works OK. Other dialogs there seem to work OK all the time.
I expect the whole FileMaker window to redraw.
Only standard elements redraw, the FileMaker client area does not.
My own plug-in is a somewhat unusual mix of technologies; it embeds Python and calls windows API functions using a foreign function interface library (ctypes). I don't use MFC; I suspect this can be the problem.
I haven't found an workaround so far, but it seems other developers did found it, because their plug-ins work OK. I'm trying to contact them too, but haven't received any answer yet. I don't think this is a real 'bug', but I don't understand what am I overlooking; I'd really appreciate some advice.