1.4.1 x64: twain Source and windows messages

[hvdev]

XnView: MP 1.4.1 - 64 bit
OS: Windows 64bit

Hello. I am writing own twain Source and have a problem with windows messages transferring from Source -> xnview -> Source.
According to TWAIN-2.4-Specification, page 645, the application must pass all messages to the source (in enabled state).
With XnView MP x64 the source does not receive messages what sent to the app handle.

To reproduce:
1. install my modified sample Source to windows/twain32/
2. enable the Source
3. press PostMSG button

Actual behaviour (bug): nothing happened

Expected behaviour: messagebox will appear (screenshot from XnView MP 32bit)

[xnview]

could you try this version?

[hvdev]

Works as expected, the issue doesn't reproduce.

[hvdev]

The issue is still present in version 1.7.2