Pressing only Shift/Alt triggering the shape to get highlighted while using Shape Builder Tool.
For reference: #4266 (closed)
Note: I cannot test it as holding shift doesn't changes the cursor too in the application build by me. But I am sure if the cursor is changing in the production (the deployed application) then this logic for highlighting the shape will work too.
Operating System: macOS (use alt for windows)
Edited by Aman Gupta