We have a Blazor Server app that uses Telerik for Blazor 3.4.0
We noticed that when we have TelerikWindow component where the Modal=true is defined, users can still use the tab button to navigate outside of the pop up. They can even access the actions and dropdowns on the site behind the TelerikWindow. We tested this on Edge, Chrome, and Firefox.
From this documentation it seems that we'd only need to set Modal="true" for a TelerikWindow component to behave similar to a TelerikDialog component: https://docs.telerik.com/blazor-ui/components/window/modal
You can even observe this behavior in the Telerik REPL:
Here is a TelerikDialog component: https://blazorrepl.telerik.com/QcOVYAGb10rFakuj32
Here is a TelerikWindow component with Modal="true": https://blazorrepl.telerik.com/wGOhYUQv27XDlK8506