For whom does the Assembly Definition Inspector fit entirely on the screen? 
My screen has 1440 vertical pixels which the Asmdef Inspector just barely fits into at full height.
Please, please … make the platforms list collapsed and ONLY expand it when we actually need it. This list is needlessly wasting precious vertical space.
And while we’re at it: please redesign the whole checkbox list because it’s very confusing. I can have “Any Platform” selected AND also “Select all” which checks all individual platform checkboxes. Or I could have “Any Platform” checked and also “Editor” which - no surprise - doesn’t make it an Editor-only asmdef (I think, I haven’t even confirmed … wouldn’t make it any less confusing though).
With “Any Platform” checked none of the other platforms should be shown to begin with. Actually, it should be a radio button with the other options being “Editor-Only Platform” and “Custom”. Only if the latter is selected should the platforms list be shown.
We really need that “editor” asmdef as a quick option, perhaps even specifically added to Create Asset => Scripting => Editor Assembly Definition.