wiki: Document invisible state

wiki/Design-Principles.md

@@ -22,3 +22,9 @@ And here are some more principles I try to follow throughout niri.
 1. Eye-candy features should not cause unreasonable excessive rendering.
     - For example, clip-to-geometry will prevent direct scanout in many cases (since the window surface is not completely visible). But in the cases where the surface or the subsurface *is* completely visible (fully within the clipped region), it will still allow for direct scanout.
     - For example, animations *can* cause damage and even draw to an offscreen every frame, because they are expected to be short (and can be disabled). However, something like the rounded corners shader should not offscreen or cause excessive damage every frame, because it is long-running and constantly active.
+1. Be mindful of invisible state.
+
+    This is niri state that is not immediately apparent from looking at the screen. This is not bad per se, but you should carefully consider how to reduce the surprise factor.
+
+    - For example, when a monitor disconnects, all its workspaces move to another connected monitor. In order to be able to restore these workspaces when the first monitor connects again, these workspaces keep the knowledge of which was their *original monitor*---this is an example of invisible state, since you can't tell it in any way by looking at the screen. This can have surprising consequences: imagine disconnecting a monitor at home, going to work, completely rearranging the windows there, then coming back home, and suddenly some random workspaces end up on your home monitor. In order to reduce this surprise factor, whenever a new window appears on a workspace, that workspace resets its *original monitor* to its current monitor. This way, the workspaces you actively worked on remain where they were.
+    - For example, niri preserves the view position whenever a window appears, or whenever a window goes full-screen, to restore it afterward. This way, dealing with temporary things like dialogs opening and closing, or toggling full-screen, becomes less annoying, since it doesn't mess up the view position. This is also invisible state, as you cannot tell by looking at the screen where closing a window will restore the view position. If taken to the extreme (previous view position saved forever for every open window), this can be surprising, as closing long-running windows would result in the view shifting around pretty much randomly. To reduce this surprise factor, niri remembers only one last view position per workspace, and forgets this stored view position upon window focus change.