I'm using v2019.2.514 and I noticed that the height of my RadWindow differs between IE and Chrome. I set a static height and do not use AutoSize.
Here's my how I define the window:
In IE developer tools, I see that the height is set to 820px and all looks fine.
In Chrome, I see that the computed to 859px.
I did search the forums and did find some posts that referred to this extra 39px but they are quite old and the response to these post note that there's an issue with Chrome and iframes but only refer to windows that are smaller than 150px. In my case, the window way beyond this threshold.