Bug reproduced on latest stable release (5.4.2) on Archlinux and Gnome with both Wayland and Xorg. When user try to move the dialog window with the titlebar, even by a single pixel, it's strangely interpreted and make totally disappear the main window until user move the dialog one to a side or to the top, at the extreme edge of the screen, the same way a user might want to resize a window using the edges. This way, main window is resized to a fullscreen one or half-the-screen depending on the edge… while there were only moved on the dialog window.
Could you please record an screencast for clarification?
He can't get screen recording to work, let's return the ticket to UNCONFIRMED for now.
Open GNOME Tweaks, switch to the Windows tab, switch off "Attach Modal Dialogs". *** This bug has been marked as a duplicate of bug 105462 ***