I just noticed that even with 4.5, I still run into this problem. Here is all I need to do to reproduce the problem.
Connect to my mac mini (Yosemite) from Ubuntu 14.04LTS and edit a swift file in Xcode 6.2.
When everything is working correctly, holding down the windows alt key (windows keyboard), will change the cursor to a “?” when you hover over a variable or pretty much any language feature.
If I make any change to the file content (insert a space for example) then when I hold down the windows alt key, rather than getting a “?” when I hover over a language feature, I get a “+”.
I have found a way to get back the correct behavior. If I simply right click in the file and then hit escape, I seem to be back to the expected behavior.