I use 2 monitors with Construct! You can right-click pretty much any window and click "pop out" or something, then you can drag it to a different window.
I usually put my "Find" results, on a 2nd monitor, but you can put your bookmarks, another event sheet, and well pretty much anything over to a 2nd monitor.
I haven't tried a ultra wide monitor but I'd LOVE to try that, and I have a feeling that C3 will adapt very well.
Pro tip if you ever make online multiplayer game: Hold ALT whilst clicking "Preview", and it will give you a 2nd preview window, which you can drag onto your 2nd monitor and observe 2 players connecting online (can keep doing ALT+Preview to make even more previews windows if needed).