I was having the same problem, especially in cshtml pages. I found this page: https://devblogs.microsoft.com/aspnet/visual-studio-11-beta-razor-editor-issue-workaround that suggests changing the indent option in Tools > Options > Text Editor > HTML > Tab to Smart instead of Block. In my case it was already set to Smart and changing it back to Block fixed the problem.
Update: I was wrong, that didn’t fix the dialogue, it just delayed it until I copied or pasted. What finally worked for me was to go to Tools > Import and Export Settings… > Reset all settings.
Possibly related question: Visual Studios 2010 – Asp.net MVC 4 Beta – long delays on paste and frequent crashing