A customer had written a plug-in for some application, and they found that their plug-in was unable to create a SysLink control via the
CreateWindowExW function. The same code in a standalone application works fine, but when the code is placed in their plug-in, the code fails.
Debugging showed that the call to
InitCommonControlsEx succeeded, but the
CreateWindowExW call failed with "Cannot find window class."
The customer is another victim of not keeping their eye on the activation context.
They attached a manifest to their DLL so that the call to
InitCommonControlsEx maps to the version of the common controls library that supports the SysLink control. But they did nothing to ensure that that context was active at the time they called
The customer's plug-in clearly falls into the case Adding Visual Style Support to an Extension, Plug-in, MMC Snap-in or a DLL That Is Brought into a Process. but they failed to follow the instructions provided therein (which boil down to "use isolation awareness").
From the symptoms, it appears that the host application for their plug-in does not activate a version-6 common controls manifest at the time it calls into the plug-in, which means that your attempt to create version-6 common controls will fail.
On the other hand, the standalone application probably uses the technique given in Using ComCtl32.dll Version 6 in an Application That Uses Only Standard Extensions, which activates the version-6 common controls when the process starts and leaves it active for the duration of the process.