Type something in a text box. Highlight and drag it. UI crashes instantly every time, leaving zombie processes behind.
Hmm, it's not causing crashing for me on OS X Yosemite, but on the other hand, it's not having quite the expected behavior either. Firstly, unless I click and hold still for a second before dragging, it just starts defining a new selection range. (This may be down to some native OS X text widget behavior perhaps – I see this "hold still for a moment before dragging" requirement also affecting OS X's Notes app, TextEdit, Chrome, and Safari, but not occurring in Firefox, MS Word, or TextWrangler.)
However, only in Vivaldi, if I do click and hold still for a moment, then drag, it copies the selected text to the new location, instead of relocating (cutting and pasting) it.