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mutter/doc/mutter-focus.txt
Sebastian Wick 50936e9953 docs: Prefix code documentation files 
with the subsystem they belong to. This makes it easier to see which
files belong where.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3485>
2024-01-03 22:41:07 +01:00

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To make choice of focus window consistent for each focus method, a
number of guidelines should be followed. (For purposes of discussion
here, I'm excluding things like the panel and the desktop from
"windows". It is technically incorrect to do this, but I'm lazy and
"windows" is shorter than something like "normal windows". See the
end of the discussion for how these special cases are handled.) The
basics are easy:
Focus method Behavior
click When a user clicks on a window, focus it
sloppy When an EnterNotify is received, focus the window
mouse Same as sloppy, but also defocus when mouse enters DESKTOP
window
Note that these choices (along with the choice that clicking on a
window raises it for the click focus method) introduces the following
invariants for focus from mouse activity:
Focus method Invariant
click The window on top is focused
sloppy If the mouse is in a window, then it is focused; if the
mouse is not in a window, then the most recently used
window is focused.
mouse If the mouse is in a non-DESKTOP window, then it is focused;
otherwise, the designated "no_focus_window" is focused
However, there are a number of cases where the current focus window
becomes invalid and another should be chosen. Some examples are when
a focused window is closed or minimized, or when the user changes
workspaces. In these cases, there needs to be a rule consistent with
the above about the new window to choose.
Focus method Behavior
click Focus the window on top
sloppy Focus the window containing the pointer if there is such
a window, otherwise focus the most recently used window.
mouse Focus the non-DESKTOP window containing the pointer if
there is one, otherwise focus the designated
"no_focus_window".
Note that "most recently used window", as used here, has a slightly
different connotation than "most recent to have keyboard focus". This
is because when a user activates a window that is a transient, its
ancestor(s) should be considered to be more recently used than other
windows that have had the keyboard focus more recently. (See bug
157360; this may mean that the alt-tab order should also change
simultaneously, although the current implementation does not do that.)
Also, sometimes a new window will be mapped (e.g. unminimizing a
window or launching a new application). Most users want to interact
with new windows right away, so these should typically be focused.
This does conflict with the invariants for sloppy and mouse focus
modes, so this wouldn't be true for a strict-pointer-focus mode. For
all other modes (non-strict-pointer-focus modes), there are only two
cases in which a new window shouldn't be focused:
1) If the window takes a while to launch and the user starts
interacting with a different application, the new window should
not take focus.
2) If the window that will appear was not launched by the user
(error dialogs, instant messaging windows, etc.), then the window
should not take focus when it appears.
To handle these cases, Metacity compares timestamps of the event that
caused the launch and the timestamp of the last interaction with the
focused window. (Case 2 is handled by the application providing a
special timestamp of 0 for the launch time, which ensures that the
window that appears doesn't get focus)
If the newly launched window isn't focused, some things should be done
to alert the user that there is a window to work with:
1) The _NET_WM_DEMANDS_ATTENTION hint should be set
2) If the new window isn't modal for the focused window, it should
appear below the focused window so that it doesn't obscure the
focused window that the user is interacting with.
3) If the new window is modal to the focused window, the currently
focused window should lose focus but the modal window should
appear on top.
Additionally, the user may decide to use the keyboard instead of the mouse
to navigate between windows (referred to as "keynav"). This poses no
problems for click-to-focus (because the same invariant can be
maintained), but for sloppy and mouse focus it requires extra work to
attempt to handle the INHERENTLY CONFLICTING CONSTRAINTS. Metacity does
this by having a mouse_mode boolean used to determine which of the two
sets of invariants holds. This mode is set according to which method was
most recently used to choose a focus window:
1) When receiving EnterNotify events from mouse movement, set
mouse_mode to TRUE.
2) When using keynav to choose a focus window (e.g. alt-tab, alt-esc,
alt-f2, move-window-to-workspace keybindings), set mouse_mode to FALSE.
3) When handling events that don't choose a focus window but rather need
a focus_window chosen for them (e.g. switch-to-workspace keybindings),
don't change the mouse_mode and just use the current value.
Note that grabs present a special case since they can generate EnterNotify
and LeaveNotify events without using the mouse, thus these events should be
ignored when the crossing mode is NotifyGrab or NotifyUngrab. THIS
MOUSENAV/KEYNAV MODERATION METHOD IS NOT PERFECT--there are corner cases
when trying to mix-and-match between mousenav and keynav simultaneously
that cause problems; but it appears to be the most reasonable tradeoff and
works well in most cases, especially if the user sticks to just mousenav
for a long time or just keynav for a long time.
Finally, windows of type WM_DOCK or WM_DESKTOP (e.g. the desktop and
the panel) present a special case, at least partially due to the lack
of decorations. For WM_DESKTOP windows, we only focus them if the
user explicitly requests it (e.g. clicks on the window, uses
Ctrl-Alt-Tab to navigate to it, uses a keybinding to show the desktop,
etc.). For WM_DOCK windows, we do not focus unless we receive a very
explicit request (e.g. Ctrl-Alt-Tab or a _NET_ACTIVE_WINDOW message;
not normal clicks).
To read more about the bugs that inspired these choices:
- When a focused window becomes invalid and another should be chosen
http://bugzilla.gnome.org/show_bug.cgi?id=135810
- When a new window is mapped
http://bugzilla.gnome.org/show_bug.cgi?id=118372
Also, the EWMH spec, especially the parts relating to _NET_WM_USER_TIME
- Modal vs. non-modal dialogs that get denied focus when mapped
http://bugzilla.gnome.org/show_bug.cgi?id=151996
- Mousenav vs. Keynav in mouse and sloppy focus modes
http://bugzilla.gnome.org/show_bug.cgi?id=167545
http://bugzilla.gnome.org/show_bug.cgi?id=101190
http://bugzilla.gnome.org/show_bug.cgi?id=357695
- Not focusing panels
http://bugzilla.gnome.org/show_bug.cgi?id=160470
http://bugzilla.gnome.org/show_bug.cgi?id=120100
There were many bugs which had to be fixed to get all the above
working; they helped form these policies and/or show the difficulties
in implementing this policy (my apologies in advance for producing a
list heavily lopsided to what I've done; it's just that these bugs are
the ones I'm the most familiar with):
bug 72314 ignore LeaveNotify events from grabs
bug 82921 focus windows on map
bug 87531 only show focus for sticky windows on active workspace (pager)
bug 94545 focus window on workspace switch is non-deterministic
bug 95747 should ignore EnterNotify events with NotifyInferior detail set
bug 97635 sticky windows always keep focus when switching workspaces
bug 102665 a window unminimized from the tasklist should be focused
bug 107347 focus windows that manually position themselves too
bug 108643 focus in MRU order instead of stack order
bug 110970 moving a window to another workspace loses focus
bug 112031 closing a dialog can result in a strange focus window
bug 115650 add _NET_WM_USER_TIME support to gtk+ (see also 150502)
bug 120100 panel shouldn't be focused after workspace applet usage
bug 123803 need final EnterNotify after workspace switch (see also 124798)
bug 124981 focus clicked window in pager only if on current workspace
bug 125492 catch the xserver unfocusing everything and fix its braindeadedness
bug 128200 focus correct window on libwnck window minimize (see 107681 too)
bug 131582 fix race condition on window minimize/close
bug 133120 wrong window focused when changing workspaces
bug 135024 _NET_ACTIVE_WINDOW messages need timestamps
bug 135786 middle-clicking on focused window to lower it should defocus too
bug 136581 window minimization vs. activation for mouse focus
bug 144900 fix focus choice on "un-showing" the desktop
bug 147475 don't lock keyboard on workspace change
bug 148364 DEMANDS_ATTENTION support for metacity & libwnck (and other stuff)
bug 149028 focus-stealing-prevention for metacity-dialog (and other stuff)
bug 149366 windows denied focus on map occur in wrong order in alt-tab list
bug 149543 consistent focus window when unshowing desktop
bug 149589 race in focus choice from libwnck messages
bug 150271 make sure "run application" dialog gets focused
bug 150668 update gtk+ _NET_ACTIVE_WINDOW support
bug 151245 application startup notification forwarding (partially rejected)
bug 151984 Soeren's idea--backup timestamp when startup notification not used
bug 151990 prevent focus inconsistencies by only providing one focus method
bug 151996 modal dialogs denied focus should not be lowered
bug 152000 fix race on window close followed by rapid mouse movement
bug 152004 ways to handle new window versus mouse invariants
bug 153220 catch the root window getting focus and reset to default window
bug 157360 focus parents of dismissed transient windows in preference to
the window that most recently had focus
bug 159257 focus the desktop when showing it
bug 160470 don't focus panels on click
bug 163450 correct highlighting in workspace switcher popup
bug 164716 refuse to focus a window with a modal transient, and focus
the transient instead
bug 166524 avoid new windows being obscured by the focus window
bug 167545 mousenav vs. keynav in mouse and sloppy focus modes
<a massive heap of bugs relating to focus stealing prevention...>
Addendum on sloppy and mouse focus
You may occasionally hear people refer to sloppy or mouse focus
modes as inherently buggy. This is what they mean by that:
1) Keynav doesn't maintain the same invariants as mouse navigation
for these focus modes; switching back and forth between
navigation methods, therefore, may have or appear to have
inconsistencies. Examples:
a) If the user uses Alt-Tab to change the window with focus, then
starts to move the mouse, at that moment the window where the
mouse is does not have focus.
b) Users expect that a workspace they previously used will not
change when the return to it. This means things like window
position and stacking order, but also the focus window.
Unfortunately, using the original focus window (which would be
the most recently used window on that workspace) will
sometimes conflict with the invariants for mouse and sloppy
focus modes. Users are much more surprised by the invariant
being broken than by having the focus window changed (see bug
94545 and probably others), so we maintain the invariant.
This only matters when using Ctrl-Alt-Arrow to switch
workspaces instead of clicking in the workspace switcher, so
this really is a keynav vs mouse issue. Either that, or a
windows-are-being-mapped exception. ;-)
c) Opening a menu, then moving the mouse to a different window,
and then pressing escape to dismiss the menu will result in
the window containing the mouse not being focused. This is
actually correct behavior (because pressing escape shows that
the user is using key navigation to interact with the window
containing the menu) but is one of those hard-to-get-right
keynav and mouse focus mixture cases. (See bug 101190 for
more details)
d) Similar to (c), moving the mouse off the menu doesn't immediately
focus the window that the mouse goes over, due to an application
grab (we couldn't change this and wouldn't want to, but
technically it does break the invariant).
e) If mouse_mode is off and the user does something to cause focus to
change (e.g. switch workspaces, close or minimize a window, etc.)
and simultaneously tries to move the mouse, the choice of which
window to focus is inherently race-y. (You probably can't satisfy
both keynav and mousenav invariants simultaneously...)
2) The sloppy/mouse invariants are often not strictly maintained;
for example, we provide an exception to the invariant for newly
mapped windows. (Most find that not allowing this exception is
confusing)
3) There are an awful lot of little cases to handle to get any focus
mode right, even for click-to-focus. Since mouse and sloppy
focus have sometimes been hard to even determine what correct
behavior is, it is much harder to get them completely right.
Plus mouse and sloppy focus users are a minority, decreasing the
motivation of window manager implementors to get those focus
modes right.
4) Because of -1-, -2-, and -3-, implementations are often buggy or
inconsistent and people form their opinions from usage of these
implementations.
5) Sloppy focus suffers from a bit of a discoverability problem (for
example, I have seen a scientist sit down to a computer for which
sloppy focus was in use and take a few minutes before figuring
out how window activation worked; granted the layout of the
windows in that situation was a bit unusual but it still
illustrates that sloppy focus is harder than it should be to
figure out). Mouse focus solves this problem; however, people
that have experience with other computing environments are
accustomed to being able to move their mouse outside the window
they are working with and still continue interacting with that
window, which conflicts with mouse focus.
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