MDI app not receiving WM_ACTIVATE and DWM messages from Windows 7 Taskbar

I have an MDI app written in Borland C++Builder 6 (upgrading to a newer version is not an option) and am using the ITaskbarList3 interface to integrate the MDI child windows into the Windows 7 ta开发者_Python百科skbar. Following Microsoft's documentation, each MDI window creates a proxy window to receive messages from the Taskbar. The tabs that I register with ITaskbarList3 do appear on the Taskbar, and the proxy windows do receive WM_GETICON, WM_ACTIVATEAPP, WM_SYSCOMMAND, and WM_CLOSE messages. They do not, however, receive any WM_ACTIVATE, WM_DWMSENDICONICTHUMBNAIL, or WM_DWMSENDICONICLIVEPREVIEWBITMAP messages. Without those, the feature is useless.

I thought it might be a manifest issue (the app does use SoftGems TThemeManager), so I extracted a manifest from a C++Builder 2010 app, added a "compatibility" section to mark Vista and Win7 as supported OSes, and replaced the stock manifest in the compiled executable with the new manifest. Still no luck.

Am I missing something?