I have seen others remark that Gazebo 11 dialog boxes were not visible. I have this currently with Gazebo 11.9.0 on ROS2 Galactic over Ubuntu 20.04.3 running in a VMware Fusion virtual machine on 2018 Mac Mini - OS X 11.0.1 Big Sur.

Work-Around:

If Gazebo 11 dialog box (e.g. 'save world as') does not display:

• press ctrl-alt(option)-dnarrow, then ctrl-alt(option)-uparrow

to display the dialog.

Repeat the sequence after every mouse click or characters typed to redisplay the changed dialog box.