This is a Menu Button control. Menu Button controls are not rocket-science, but this control is also a demonstration on how to draw an ownerdrawn button in both Windows 98 and Windows XP.
The control displays itself as a regular button, except that it has a Glyph (a Little arrow) on the right signalling that it displays something when you click it. All it does is to display a popup menu. The popup menu is custom drawn because I wanted to be able to add colourful icons to it. The button is also ownerdrawn because it needs to Paint the dropdown-glyph on the right.
The problem with ownerdrawn buttons is that on Windows XP and greater we need to paint it with the current theme. Before Windows XP it was simply a matter of using the BS_OWNERDRAWN style. Unfortunately this also meant that we had to repaint the entire button exterior as well. Since a button always behaved the same, we could use the DrawFrameControl() API to do much of the drawing.
Along came Windows XP and changed all that. The DrawFrameControl() no longer works and we'll have to dip into the Theme API to get the job done. Well, not quite... because the old Windows Common Controls ListView and TreeView introduced a new ownerdraw method: the NM_CUSTOMDRAW notifications. And in Windows XP the Button control inherits this behaviour too (as well as supporting the old WM_DRAWITEM-range of messages).
The trick to drawing an ownerdraw button under Windows XP is not to draw the button at all.
The Button sends WM_NOTIFY / NM_CUSTOMDRAW notifications to the parent. If we wanted to drastically change the look of the button, we would start to custom paint here but since we just want to add an icon and some visual decorations, we'll answer the notification with the CDRF_NOTIFYPOSTPAINT code. This instructs Windows to paint the button entirely. When done, Windows calls us again with NM_CUSTOMDRAW and allows us to paint on top of the button image.
