Visual Studio 2017 (VS2017) 'find in files': In the search tool, the "Look in" option "all open documents" option seems to be missing from the dropdown.
I had found that option very handy in the past.
This option does work in my SSMS (17.7)
NOTE: I subsequently found that this had to do with editing .rdl files for SSRS projects in design-view, see my own answer below, if interested.
For Visual Studio 2017, have you tried 'Find and Replace'? (ctrl + shift + f)
I'm referring to this, inside 'Look In' drop down, you can select the scope.