When opening a dialog by using IDialogService the dimmed background is only visible on the current page. If e.g. we have a TabbedPage the tabs-area is not disabled nor greyed out. Therefore the user can switch tabs.
Is there a possibility to prevent this behavior?
All the background is gray and the user should not be able to select the other tab.
The currenty page is gray only and the user is able to switch tabs,

this will not be supported in the super near term as we are blocked by Xamarin.Forms... however I just spoke with @PureWeen it seems that xamarin/xamarin.forms#6558 from @andreinitescu should allow us to modify the way that we handle Dialogs to use an actual Modal page that will properly mask a Tab Bar, Navigation Bar, or a Master Detail Page.
I should note that the best way to work around this issue will be from the upcoming release of Prism.Plugin.Popups which will have an implementation that injects your Dialog Content inside of a PopupPage this will properly overlay the Navigation bars in TabbedPages / NavigationPages
moving this to roadmap... the original PR we were waiting on was closed, however the work we needed has been merged and should be available in the next release of Xamarin.Forms.
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this will not be supported in the super near term as we are blocked by Xamarin.Forms... however I just spoke with @PureWeen it seems that xamarin/xamarin.forms#6558 from @andreinitescu should allow us to modify the way that we handle Dialogs to use an actual Modal page that will properly mask a Tab Bar, Navigation Bar, or a Master Detail Page.