This is one for the PowerApps Product team.
I work on different client tenants, using Edge. I have observed the following phenomenon in different tenants.
PowerApps Designer is running in Edge, displaying on a secondary monitor that uses a 1920 x 1080 resolution at 100% scale.
When the Edge window is maximised, the publish button at the far right of the top tool bar is not visible. The right-most icon is the Save button with the dropdown.
When I restore the Edge window to a smaller (!) size, the publish button appears again.
When I then drag the side of the Edge window to make it wider, the Publish button disappears when the window reaches a specific size.
At a window width of 1886, the Publish button is still visible.
At a window width of 1887 and higher, the Publish button disappears.
If I stretch the window across two monitors, I can see the publish button return. It looks like the container for the tool bar has been cut off and is wider than the window bounding box.
This is clearly a bug in the UI programming, some CSS setting needs to be adjusted. On a 1920 x 1080 monitor with the window maximized, the button will never show.
this is an animated GIF of the window resizing effect.