I would like to present you in a few sentences the purpose of the RadDeviceDetectionFramework, in order to give you a better understanding of this product and its features.
It is designed to distinguish whether your application is browsed on a desktop screen or on a mobile device screen. Its purpose is to allow automatic enabling of controls' Mobile render mode when browsed under mobile devices as well as to allow you to apply different CSS for different predefined or custom ranges of mobile devices' screen size. Desktop screens size is not covered by this tool as those screens are big enough to allow work with the controls' Classic or Lightweight render mode.
Now let's focus on the specific questions:
1) As stated in the RadDeviceDetectionFramework, ExtraLarge would definitely mean that the screen size is over 1366 CSS pixels. The majority of desktop screens fall into that category nowadays. Each range corresponds to the specific CSS pixels range specified in the documentation.
The RadDeviceDetection purpose is to enforce render mode Mobile for a control with RenderMode="Auto", when browsed on mobile device. And this is achieved not only by accessing the screen size, but the OS used as well. Thus, a RadMenu would not become mobile unless it is browsed on a mobile device - smaller desktop browser windows would be ignored as they would be detected as not mobile based on OS.
2) RadPageLayout has different viewport breakpoints than the ones used for RadDeviceDetectionFramework due to its different purpose. The RadPageLayout is used for setting up layout, not for enabling render mode. Thus, it needs one more level for enabling different layout for different cases. In addition, the RadDeviceDetectionFramework provides the opportunity to define your own custom ranges, if the default ones does not fit your case.
3) Using the browser version info in the agent string to determine if the browser on the device supports media queries
is not supported by the RadDeviceDetectionFramework as it goes out of its current purpose scope. Using such info for browser features could be dubious as browsers tend to present themselves as other browsers due to the so called browser wars
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