I initially raised the following issue about your Cloud version via email Mar 2019:
I don’t get a contextual menu when I “right-click” (i.e. CTRL-click on Mac) on items on the canvas.
I do get contextual menus in the left-hand-pane (the wireframes list), but not anywhere on the canvas.
I did get replies, and we tried various things, and eventually gave up (maybe it was my OS, old, or something peculiar to my setup).
I recently bought a new iMac, am running the latest OS, etc. And now I use Google Drive version of Balsamiq. Same problem.
I thought I’d post in the forum and see if anyone else has got this bug.