Select elements are unusable with the newest Opera Developer and more than one monitor. They sometimes appear on another monitor, or just don't have a selection box after clicking on them. I can still use the up and down arrows to navigate through the select options.
In this set up I have a 4K monitor all the way to the left, a 1080p monitor rotated 90 in the middle and a 1080 on the right.
When I disconnect all but one monitor the selects work as expected.