Example Solution uploaded as .jpg from .zip
The provided example has various controls in different tabs (including a scrollviewer). As you should be able to see in the fluent theme when using the scroll wheel to scroll the tab header an item in the tab header has to be focused. If I click a button, or select a textbox in one of the tabs I can no longer mouse over the tab header to scroll the tabs (I have to click on a tab to give it focus for the scroll wheel to work again).
This is not the case the other way around (Tab 5 or tab 6). If the tab header is focused the scrollViewer in the tab will still scroll on mouse over without focus being granted to it.