I have this problem persisting over several months. I have used and written about Report Builder a number of times (in fact two books). I never had any problem, but after entering the Insider program, Report Builder 3 never worked as it should.

I have posted this to MSDN Reporting Services forum, but after a long conversation they asked me to seek your help.
Some previous links from last year when this problem came up;
This was the Report Builder used.
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Recently I downloaded the latest version of the Report Builder. The problem remains the same. The UI is all messed up and useful buttons / Controls missing in the Expression Builder like I described in my posts.
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