COM host object usage vs postMessage stability

[reynoldslabware]

I have a DLL which injects a java script function used to call the DLL.
In scenario 1 I use postMessage to send a string to the DLL.
In scenario 2 I use the host COM object approach to call the DLL.
The postMessage approach runs well.
When using the COM approach, our application is getting windows messages
with message ID 1025 (WM_USER + 1), 0 for wParam and 0 for lParam.
The class of the associated window is Chrome_MessageWindow.
Calling the usual TranslateMessage() and DispatchMessage() results in a crash.
Can anyone shed light onto these Windows messages please?