I have tested your scenario on both desktop and iPad Safari and unfortunately I was not able to reproduce the behavior described. On both type of browsers I observe the same window rendering. I am sending you screenshots for reference.
You can examine the Opening a RadWindow from within a RadWindow
help article to see another way of doing this. It describes how you could create child windows that do not depend on their parent dimensions and can be freely dragged around the same way their parent can. I am sending you a sample project showing that behavior and a screenshot of it from an iPad Safari.
If this approach does not provide a solution for you, I would suggest that you provide us with a very simplified runnable project that demonstrates the behavior that you describe. It may include aspx, code behind, web.config, as well as any other files you find necessary. Thus, we could test it locally and determine what might be causing the issue that you are experiencing.
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