The page scrolls instead of the window component moving.
You can drag it a few pixels at a time at most.
EDIT:
A workaround could be to disable touch events for dragging (which may also help for other components like the Splitter):
<TelerikWindow Visible="true" Class="workaround-for-touch-drag">
<WindowTitle>the title</WindowTitle>
<WindowContent>the content</WindowContent>
</TelerikWindow>
<style>
.workaround-for-touch-drag {
touch-action: none;
}
</style>