There are two main use cases for $SUBJ: 1) when app in fullscreen crashes, it leaves user with bad resolution - for inexperienced user, there's no obvious way how to get back. When this is fixed, X should return automatically to original display settings. 2) when app resizes root window, it makes all panels, desktop managers, window managers etc. change layout. Some app layouts (gnome-panel) are likely to break then, some are just incovenient (like when you have a large window under a smaller one that goes fullscreen, larger window gets displaced to the second monitor).
Unfortunately, there's no way we can do this due to a number of reasons. The root window size forms the base for all window and drawing clipping, and the primary way we notify applications (including your screensaver, etc) of resolution changes. There may be better way to do temporary resolution changes, but we really can't avoid resizing the root window.
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