The blur pipeline is still cached on ClutterBlurEffect, and we
simply update the uniforms when asked to create the pipeline.
Now that ClutterOffscreenEffect will use the blur pipeline, it
doesn't need to override the paint_target() vfunc anymore.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1355>
The most annoying aspect of ClutterOffscreenEffect right now, and
the reason for all its subclasses to override pre-paint, is that
the pipeline creation isn't under subclasses' control. That means
all subclasses must ask ClutterOffscreenEffect to run pre-paint
and create the pipeline, then they all create their own pipelines
to paint.
To reduce this complexity, add a new create_pipeline() vfunc to
ClutterOffscreenEffect. Next commits will port effects to use this
new vfunc.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1355>
ClutterLayerNode currently skips pushing the offscreen framebuffer when
no operations are set. This was added at the time because pushing and
popping was a synchronization point in Cogl, slow enough to force the
layer node to have this protective measure. Nowadays, pushing and
popping on the paint context is free.
Make ClutterLayerNode always push and pop in pre and post paint.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1355>
ClutterBlurNode is a paint node based on ClutterLayerNode
that draws all children in an offscreen framebuffer, blurs
this framebuffer, and finally paints the blurred contents
according the the paint operations added.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1627>
Copy-paste error sneaked ClutterTransformNodeClass in
clutter_blit_node_class_init(). It wasn't problematic
because they both typedef to ClutterPaintNodeClass, but
fix it anyway.
Also switch to g_object_ref() in clutter_blit_node_new().
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1627>
Until now we would `clutter_input_focus_set_cursor_location` with
cursor-rectangle-in-physical-pixels + actor-location-in-stage-coordinates.
But those use different scaling factors so it only got the right answer
when the framebuffer scale was 1.0.
This directly determines the geometry of the invisible dummy cursor in
gnome-shell ibusCandidatePopup.js:
```
panelService.connect('set-cursor-location', (ps, x, y, w, h) => {
this._setDummyCursorGeometry(x, y, w, h);
});
```
And because it's invisible it wasn't obvious that it was wrong until you
enable `CLUTTER_PAINT=damage-region` and you can see its ghost at the wrong
offset and scale.
So now we `clutter_input_focus_set_cursor_location` using purely unscaled
stage coordinates. And `CLUTTER_PAINT=damage-region` shows that
gnome-shell's `_dummyCursor` is placed precisely over the visible cursor.
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3399
and probably other IBus issues that arise when using framebuffer scaling.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1576>
`priv->cursor_rect` is stored in physical pixels, not local coordinates.
So unscale it before returning from `clutter_text_get_cursor_rect`, which
is explicitly documented as returning "actor-relative coordinates".
This went missed with fractional scaling support, and unnoticed as nobody
uses `clutter_text_get_cursor_rect` yet. But that will soon change.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1576>
Before now it was assumed that the only negative local coordinates would
be those introduced by `_clutter_actor_box_enlarge_for_effects`, and we
used the difference for `fbo_offset_x/y`. But that was misguided (of me)
because gnome-shell can give us paint volumes at negative coordinates too,
like when rendering `box-shadow` on the top or left edge of an actor.
The maximum extents of negative coordinates we might need to render are
in fact the (enlarged) left and top edges of the paint volume. So use
those as the FBO offset. This places the actor's local origin correctly
within the FBO and thus also ensures it's not over-clipped at the edges
of the FBO, which now line up with the enlarged extents of the paint
volume.
This fixes one third of
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1090
The other fixes required are !1053 and gnome-shell!1417.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1538>
Apparently it can happen that a timeline tries to pick a frame clock
from an actor that's on a stage, but the actor still doesn't find a
frame clock and returns NULL.
This probably is the case when starting a timeline right after attaching
an actor to a newly created stage, so before the first stage-update
cycle. In this case clutter_actor_update_stage_views() will not have run
and the stage-actor will have priv->stage_views set to NULL even though
there are stage views.
To prevent this from happening, use the complete list of stage views
maintained by the backend when picking a frame clock for the stage.
This doesn't fix any issue appearing on master, but is correct
nonetheless.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1631>
Instead of using native backend platform data specifically, store
this info in ClutterMotionEvent. This includes time in usec since
it's just used for motion events, in the future it could make sense
to make these general to all events again, but it could make sense
to make ClutterEvent structs private before.
In order to express that a motion event has relative motion info,
the CLUTTER_EVENT_FLAG_RELATIVE_MOTION event flag has been added
for it.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1623>
We had code in both backends that sort of independently associated
sequences to slots. Make both transform slots to sequences the same
way, so they may share the implementation convert those back to slots.
This helper now lives in Clutter API.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1623>
We have this as platform-dependent data in the native backend, and
a bunch of fallback code done in place in the evcode users. Stop
making this platform-dependent data, and move it to the relevant
ClutterEvents.
The fallback code for the X11 backend case is about the same, but
now it is done directly by the X11 backend.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1623>
Making this an event is overly convoluted, accounting that we
emit the event, then convert it to a ClutterStage signal, then
its only consumer (a11y) sets the active ATK state.
Take the event out of the equation, unify activation/deactivation
of the stage in MetaStage, and use it from the X11 backend too.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1623>
It is not possible to express a blit operation using paint
nodes as of now. This is a requirement for GNOME Shell, e.g.,
to implement its blur effect.
Add a new ClutterBlitNode node that takes two framebuffers as
input, and blits the source framebuffer into the current one
according to added rectangles.
Because this paint node uses the rectangles in a different way
compared to all the other nodes, add an auxiliary method to
ensure all blit operations are valid.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>
ClutterLayerNode is the "offscreen framebuffer" node, that paints it's
child nodes in a separate framebuffer, and then copies that framebuffer
to the parent one.
It'll be useful to hand ClutterLayerNode which framebuffer and pipeline
to use, as this is a requirement for porting e.g. ClutterOffscreenEffect
and subclasses.
Add a new clutter_layer_node_new_to_framebuffer() API.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>
Some effects, such as ShellBlurEffect and ClutterOffscreenEffect, need
to make sure the actor is painted fully opaque. With ClutterActorNode,
however, that is currently not possible.
Add a new 'opacity' parameter to clutter_actor_node_new(). It follows
the opacity override heuristic, where -1 means disable, and anything
else is clamped to [0, 255].
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>
Introduce a new paint_node vfunc that, if implemented, allows
the effect to add nodes to a transient paint node that is
immediately painted. This is a transitional step until we
have fully delegated paint node rendering.
The most basic implementation of a ClutterEffect.paint_node
vfunc, and also the default implementation, is with an actor
node, as follows:
```
static void
foo_bar_paint_node (ClutterEffect *effect,
ClutterPaintNode *node,
ClutterPaintContext *paint_context,
ClutterEffectPaintFlags flags)
{
g_autoptr (ClutterPaintNode) actor_node = NULL;
actor_node = clutter_actor_node_new (effect->actor);
clutter_paint_node_add_child (node, actor_node);
}
```
This example gives the exact same behavior of simply calling
clutter_actor_continue_paint(). In the future, the paint node
itself will be a parameter of clutter_actor_continue_paint()
and we'll be able to simplify it event more.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>
ClutterEffectNode is a private ClutterPaintNode implementation
that does effectively nothing, but helps organizing the paint
node tree. It also helps debugging, since it can output the
effect class and name to the JSON debugging routines.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>
The idea of having a paint node tree is that we don't really need
to retrieve the framebuffer from ClutterPaintContext. For example,
ClutterLayerNode draws into an offscreen framebuffer; if any child
of a layer node needs to retrieve a framebuffer to draw, the layer
node's offscreen framebuffer should be used.
However, clutter_paint_node_get_framebuffer() goes straight to the
root node of the tree, skipping any potential paint nodes with a
custom framebuffer.
Modify clutter_paint_node_get_framebuffer() to walk up the paint
node tree until a node with a custom framebuffer appears. In many
cases, this will end up either in dummy or layer node's custom
framebuffer implementations.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>
Fixes a compiler warning with -Wmaybe-uninitialized enabled:
../../../../Source/gnome/mutter/clutter/clutter/clutter-actor.c: In function ‘clutter_actor_paint’:
../../../../Source/gnome/mutter/clutter/clutter/clutter-actor.c:3808:50: warning: ‘result’ may be used uninitialized in this function [-Wmaybe-uninitialized]
3808 | else if (result == CLUTTER_CULL_RESULT_OUT && success)
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
Which might presumably happen in the unlikely case that there's no clip
frusta.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1624>
Instead of going through the event queue, stage handling code, and
back to the input device via a vmethod call, do this directly in the
MetaSeatImpl. This is not too different from X11, where everything
happens inside the backend.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>
All that is left in the "public" struct is all state that ClutterStage
delegates on ClutterInputDevice. That should move somewhere else, but
not here, not now.
All private fields belong to construct-only properties, with only getter
API, and idempotent vmethods (except keyboard a11y, atm). This should
be enough to make ClutterInputDevice obviously thread safe, outside the
backend.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>
Depending on the backend, we want to integrate this object at different
levels. It will sit close to the MetaBackendX11/MetaSeatX11 in X11, but
it will be put deep down with MetaSeatImpl in the native backend, in a
separate thread.
Since we can't depend on a single object type, nor are able to track
ClutterSeat signals neatly, make this API something to be called
explicitly by backends.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>
This will query a device state; position and modifiers.
Note that ClutterSeat subclasses don't implement the vmethod yet,
so calling clutter_seat_query_state() may crash ATM.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>
Testing points and rays against boxes is substantially cheaper - in
fact, almost trivial - compared to triangles. Check if the actor's
paint volume doesn't intersect with the current pick point / ray,
and skip recursing altogether in those cases.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1520>
The cached pick mode never actually cached anything, since it is
always, unconditionally reset when painting. Furthermore, next
commits will cull out actors during pick, which makes the pick
stack uncacheable.
Remove the cached pick mode.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1520>
Users of Debian arm64 (aarch64) report that on at least some GPUs
or screens, after time-based screen blanking has occurred, it is not
possible to unlock the screen. Bisection indicates that this regressed
in commit 209b1ba3, so presumably this is because a refresh rate of 0
is reported while the screen is blanked, leading to the frame clock
pausing forever.
Fixes: 209b1ba3 "clutter/frame-clock: Adapt refresh rate from to frame info"
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1536
Bug-Debian: https://bugs.debian.org/974172
Signed-off-by: Simon McVittie <smcv@debian.org>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1601>
It's currently possible that some last_paint_volumes don't get updated
during a paint cycle, this can happen when a ClutterOffscreenEffect is
used: The offscreen effect might skip painting the content and the
children of an actor because it uses its own offscreened texture
instead. This means the offscreen effect doesn't call
clutter_actor_continue_paint(), and thus the the last_paint_volumes of
the children won't be updated.
Now one might think that isn't a problem, because as soon as a child
changes it's size or position, the offscreened texture would get
invalidated and clutter_actor_continue_paint() would get called. It's
not that easy though: Because the last_paint_volume includes all the
transformation matrices up to eye-coordinates, it has to be updated on
any changes to matrices, which includes position/transformation changes
to any actor up the hierarchy.
Now that's where get into problems with the offscreen effect: In case of
transformation changes to the offscreened actor or an actor up the
hierarchy, the offscreened texture won't get invalidated (that makes
sense, we can simply paint it transformed) and the last_paint_volumes
won't get updated even though they should.
This leaves us around with outdated last_paint_volumes where
last_paint_volume_valid is still set to TRUE. It can cause issues with
culling and clipped redraws.
So fix that by ensuring that all children that would get painted by
Clutter get their last_paint_volumes updated in case a ClutterEffect
decided not to call clutter_actor_continue_paint().
This ignores the case where a paint() vfunc override does the same and
doesn't call clutter_actor_paint() on children. Let's ignore this case
for now, there shouldn't be any implementation which does that and
ideally in a world that's painted solely by ClutterContent, we can get
rid of that vfunc in the future.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1591>
The first comment isn't really needed anymore since
the is_full_stage_redraw_queued() underneath is quite self-explaining.
Also rephrase the second comment a bit, including that
_clutter_paint_volume_get_stage_paint_box() does the aligning to the
pixel grid.
Finally, the last comment also looks out of date since we do that
rounding inside _clutter_paint_volume_get_stage_paint_box(), so remove
it.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1511>
We currently pass actors a reference to their associated
ClutterStageQueueRedrawEntry when queueing a redraw. This "splitting" of
the ownership of the entry has introduced quite a few bugs in the past
and is hard to follow.
So give up the "splitting" of the ownership and exclusively handle those
entries inside ClutterStage. To still allow removing the entry when an
actor gets unrealized introduce clutter_stage_dequeue_actor_redraw()
similar to what we already have for relayouts.
To be able to efficiently find entries when actors queue redraws, make
pending_queue_redraws a GHashTable, which fits quite nicely and also
allows removing the QueueRedrawEntries actor pointer in favour of the
key of the hashtable.
Since the struct is now private to ClutterStage, we can also rename it
to QueueRedrawEntry.
While at it, also sneak in the removal of the leading underscore from
clutter_stage_queue_actor_redraw().
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1511>
The "queue-redraw" signal is not used anywhere in Clutter and we now
also removed the vfunc implementation of the stage. So stop emitting it
and remove it, but keep the propagate_queue_redraw infrastructure to
make sure clones still get their redraws queued.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1511>
Since we now decoupled the "queue-redraw" signal from creating the stage
clip, we can move signal emission into
_clutter_actor_queue_redraw_full() and emit the signal right away when
queueing a redraw on an actor. With that we now no longer have to
accommodate for the stage pending_queue_redraws list changing while
iterating over it.
To ensure we don't emit the signal too often when multiple redraws are
queued on one actor, use the propagated_one_redraw flag to limit the
number of emissions to a single one for every update cycle.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1511>
Putting together the redraw clip of the stage never really fitted nicely
with the "queue-redraw" signal emission, it forces us to emit the
signals in a batch and we also use a weird trick to get the old paint
volume that's already on-screen into the final redraw clip (we call
_clutter_actor_propagate_queue_redraw() on the stage).
So start breaking up this association by making the stage explicitely
request the redraw clip from the actor and removing the
ClutterPaintVolume argument from _clutter_actor_finish_queue_redraw().
This is done by adding a private function
clutter_actor_get_redraw_clip() which returns our old (currently
visible) paint volume and the new paint volume.
This also allows removing the check whether a full stage redraw has been
queued in clutter_actor_real_queue_redraw() and we can now just stop the
signal emission if a propagation happened at least once.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1511>
The most common case for Clutter is 2D axis-aligned actors, which
maintain these properties even after projecting to eye coordinates.
In those cases, we can use a simpler hit test by checking against
boxes.
Not only this is simpler, but this maintains an important aspect
of picking that is a requirement for Clutter: watertightness. Even
though the triangles checks do work on x86_64, they do not guarantee
watertightness. This breaks tests on ARM.
Use graphene_box_t to hit-test axis-aligned 2D actors.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1599>
clutter_actor_paint() implements a clear preference for custom clips
over clip_to_allocation: If a custom clip is set, clip_to_allocation is
ignored.
Since the paint volume reflects what Clutter is going to paint, we
should handle it the same when putting together our paint volume: So
first handle custom clips, and if one is set, use that. Then handle
clip_to_allocation, and if that's set, use that. And finally, if both
aren't set, union our allocation with the children paint volumes to get
the building volume.
clutter_actor_paint() also doesn't check whether the custom clip is
empty: If that's the case, it will simply not paint anything. Given that
that's allowed by clutter_actor_paint(), the paint volume should also
follow here and return an empty paint volume in case the custom clip is
empty.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1592>
When a custom clip is set for an actor, this actor is not going to allow
any painting outside that clip. That includes the children, which may
also not paint outside that clip.
Now in case clip_to_allocation is set to TRUE, we already already do the
right thing and simply use the allocation as our paint volume, ignoring
the volumes of our children. The same should be done for the custom
clip, so also stop the process of building the paint volume once we see
that a custom clip is set and simply use that clip.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1592>
When using CLUTTER_DEBUG_PAINT_DAMAGE_REGION, the blue swap region is
always a superset of the red redraw region. So painting both in full (since
the previous commit) just meant the red region was overdrawn and came out
purple. That doesn't provide enough visual contrast, changes the user
experience unexpectedly and reduces performance.
So just subtract the redraw region from the swap region. This way the
red redraw region is always red, not purple.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1571>
Previously when CLUTTER_DEBUG_PAINT_DAMAGE_REGION was set, that would lead
to has_buffer_age==FALSE, which would lead to use_clipped_redraw==FALSE
which would mean swap_region was always empty. And so the blue region of
CLUTTER_DEBUG_PAINT_DAMAGE_REGION was always empty, *and* fb_clip_region
was always the full view rectangle which is not useful for debugging.
Now when CLUTTER_DEBUG_PAINT_DAMAGE_REGION is set, we don't let that
affect use_clipped_redraw, which means fb_clip_region is calculated
realistically.
But that's not enough. Calculating fb_clip_region properly with
CLUTTER_DEBUG_PAINT_DAMAGE_REGION would still lead to colouring artefacts
left on screen from previous frames that don't apply to the current frame.
So to fix that we also paint_stage for the whole screen every time when
using CLUTTER_DEBUG_PAINT_DAMAGE_REGION.
So now you will only ever see red and blue shading that's applicable to
the current frame, and no artefacts from the previous frames.
Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1535
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1571>
It was being painted without scaling and offsetting so would only look
right at scale 1.0 and only on the view with origin (0,0). Now we include
the framebuffer scale and view origin it will be painted in the correct
location.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1571>
As explained in https://gitlab.gnome.org/GNOME/mutter/-/issues/1494,
with commit 29caa5bea5 we stopped queueing
a relayout for the parent of the removed actor in
clutter_actor_remove_child_internal(). This relayout was, as opposed to
the relayout in clutter_actor_real_hide()/clutter_actor_real_unmap(),
queued unconditionally without looking at the parents NO_LAYOUT flag.
Now while that relayout in clutter_actor_remove_child_internal() would
do unnecessary work if the parent had the NO_LAYOUT flag set, it did
also queue a redraw of the parent, which is necessary in any case.
So by removing that relayout in clutter_actor_remove_child_internal(),
we stopped queueing redraws for NO_LAYOUT parents when a child gets
removed from the scenegraph. This caused bugs where the texture of the
child would be left visible on the screen even though the child got
destroyed.
To fix this, make sure again that we always queue a redraw on the parent
when unmapping a child.
Fixes https://gitlab.gnome.org/GNOME/mutter/-/issues/1494
The redraw_pending boolean is used to schedule the first stage update
after starting Clutter. This flag is superfluous because we have the
pending_finish_queue_redraws flag which does the same.
While at it, also remove the redraw_count debug variable, since there
should be better ways to count the number of queued redraws nowadays,
for example Sysprof marks.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1527
Queueing a redraw with a clip is easy enough and this function is
private anyway, so remove it and call _clutter_actor_queue_redraw_full()
instead in the one function using it.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1527
ClutterRedrawFlags are a way to give Clutter additional context
about what it needs to redraw. There currently is only one flag defined,
CLUTTER_REDRAW_CLIPPED_TO_ALLOCATION, this flag would clip the redraw to
the actors current allocation.
Since ClutterActor also provides the clip_to_allocation property (which
affects the paint volume of the actor instead of only one redraw), the
additional CLIPPED_TO_ALLOCATION flag seems unnecessary. It's also only
defined to be used privately in Clutter, which it never is, so let's
remove it.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1527
Turns out the g_signal_has_handler_pending() call in
update_stage_views() is actually more expensive than comparing the
sorted list (which is usually very short), so remove that and simply
always emit the signal.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1524
This commit introduces a few important changes in order to
acommodate graphene_ray_t. Most of them are positive changes,
so don't panic :)
The first very visible change is that neither the actor box
nor the clip rectangles are projected before being pushed.
This required changing the parameters of the related functions
at both ClutterPickContext, and ClutterPickStack, to receive
boxes instead of vertices. These rectangles are projected on
demand now, so in the best case (first actor picked) only
one projection happens; and in the worst case, it projects
as much as it does now.
The second important change is that there are no more checks
for axis-alignment anymore. That's because picking now happens
in 3D space, using triangles.
Talking about triangles in 3D space, this is what is used now
for picking. We break down each actor rectangle in 2 triangles,
and check if the projected pick point is inside any one of them,
of if the ray intersects any one of them. The same check happens
for the clip rectangles.
Checking the projected pick point is both an optimization for the
2D case, and a workaround to graphene_ray_t problems with float
precision, which is specially visible on edges such as the top
bar.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1509
ClutterPickStack is a new boxed type that stores the vertices
and clip rectangles. It is meant to be a byproduct of picking,
and takes over most of what ClutterStage currently does.
It introduces a 'seal' system, inspired by MetaKmsUpdate. After
the pick operation is done, and the rectangles are collected,
the pick stack is sealed, and is not allowed to be externally
modified anymore. Internally, it still can invalidate pick
records when an actor is destroyed.
For now, it handles both the clip rectangles, and the matrix
stack, separatedly. Future commits will rearrange this.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1509
