Fullscreen mode

A mode where all panels are removed, the window goes fullscreen, the background color turns to black.
Hovering the top side of the window would reveal the toolbar (need to check if makes sense within MD).

The issue names no files or tests. Start by locating the Qt/QML window, panel, and toolbar entry points; done means a fullscreen black-background mode hides all panels and reveals the toolbar when the window's top edge is hovered.

Written by the indexing model from the issue text.