I've just tried a sample simple window demo and it turns it that this did not occur with a registered window, which leads me to believe more now that this is an issue with Dialog Boxes only. Furthermore, I've noticed now that this occurs even when switching windows (i.e. alt-tabbing out of the dialog box also de-maximizes it). I have no idea what is causing this. My dialog box is defined as so in my resource file:
and has the following styles added to it during run-time:
IDD_DIALOG DIALOGEX DISCARDABLE 5, 0, 0, 0
STYLE DS_MODALFRAME | WS_POPUP | WS_CAPTION | WS_SYSMENU | DS_SHELLFONT | WS_MINIMIZEBOX
FONT 8, "MS Shell Dlg"
WS_SIZEBOX | WS_MAXIMIZEBOX