Thanks for the further response Svenne.
I guess given the cost of RND Portico theres a much reduced user base to elicit any comments from.
I suspect this would have to be raised direct to Steinberg via a submission other than just in the forum. I see from your link that PG the developer on Wavelab has provided some input so at least his now aware of the issue but whether it can be fixed directly within Wavelab itself may be the issue.
I'm using an apple trackpad and it appears to me that both the plugin and WL appear to react to the pointer although I suspect one reads it first but it doesn't stop the control passing through to the second program. I'd suspect its the plugin as its the window I'm working in when the problem occurs.
Its also interesting that is only the Yamaha plugins this has been noticed on which may indicate its a programming standard employed by the Yamaha programers. This is the problem with multi-national/multi brand groups you're never sure where to pitch the problem.
Cubase 7.5.10, CC121, MR816CSX, Windows 7 32&64 iMac OSX 10.9 Maverick.
Wavelab 8.03, Groove Agent 3, HSO, Halion 5, Grand 3, Halion Sonic 2, Cubasis for iPad, ICPro other plugins.
Keep smiling its later than you think.