In the latest build, Telerik.Web.UI is version 2018.2.710.45, the image manager, media manager and few other popup windows open to the right size and then go 100% in hight or more.
Using Chrome, you can see this happening the ribbon sample in the product demos. Go to https://demos.telerik.com/aspnet-ajax/controls/examples/integration/ribbonbarandeditor/defaultcs.aspx?product=editor and then click Insert followed by Image Manager. You will see it open to the correct size and then grow in height when the contents are displayed.
In my project, it grows to 100% with a strange pink striped line at top and bottom, the online product sample goes even bigger than 100% (probably a skin thing).
On every build I get, I check the image manager to see what it is doing this time and if it working right since it gets broken all the time.
Please fix and TEST properly for the next build. I do not look forward to putting out my web app with this defect.
George