Dragging windows across multiple screen glitches and hides window
When I drag the window in the normal titlebar area to another window, the styling of the window is removed, and when you let go of the window, it will disappear entirely. It does seem that processes in the disappeared window will continue to run, but there is no way to get back to the window.
A workaround I am using is dragging the window by the very thin edge where the resize cursor shows. A drag that works, followed by a drag that fails is shown in this video:

Im running the lastest release (v3.2.6beta4) on macOS Mojave 10.14.4.
The second drag you demonstrate is actually dragging a tab. Open a second tab and window to see how you can rearrange tabs and drag them to other windows.
But I do admit it’s broken and have no idea why they vanish. I haven’t had time to learn how this works to fix it.
You can actually drag the window itself above the invisible tab bar. Slightly above the tab title and below the window edge.