Like xcomposite, this was programmed to select the first display by
default. Change it to not capture any display unless explicitly selected
by the user.
There are two situations where xcomposite window capture will capture
random windows: on first creation, and when going to the properties when
the current window is invalid. The first happens because for whatever
reason someone decided to just make it capture the first top-level
window if there is no set value. The second happens because the
properties widget cannot find the value it's looking for and defaults to
the first one when the properties are opened, thus selecting and
capturing the first window in the list (which is probably something we
should fix in the properties view at some point but I don't want to dive
into code that's even *more* cursed than xcomposite code right now)
I think that this was a major oversight and that whoever wrote it
however many countless years ago did not realize that this is something
that maybe users don't want to have happen.
So instead, this diff makes it so that on first creation, it creates a
value that says "[Select a window to capture]" that keeps the capture
inactive until a user actually chooses a window rather than the
top-level window. It also makes it so that if the user has a window that
is no longer valid, it will keep that window in the list and as the
currently selected value, which prevents it from automatically selecting
the first window in the list when properties are opened.
(Have I mentioned xcomposite is cursed? Trying to debug xcomposite code
in a debugger freezes my window compositor and forces me to do a hard
restart of my entire computer, so I was forced to use printf debugging.
Absolute nightmare-inducing code in here.)
Add a new Linux capture based on PipeWire [1] and the Desktop portal [2].
This new capture starts by asking the Desktop portal for a screencapture session.
There are quite a few D-Bus calls involved in this, but the key points are:
1. A connection to org.freedesktop.portal.ScreenCast is estabilished, and the
available cursor modes are updated.
2. CreateSession() is called. This is the first step of the negotiation.
3. SelectSources() is called. This is when a system dialog pops up asking the
user to either select a monitor (desktop capture) or a window (window capture).
4. Start() is called. This signals the compositor that it can setup a PipeWire
stream, and start sending buffers.
The reply to this fourth call gives OBS Studio the PipeWire fd, and the id of the
PipeWire node where the buffers are being sent to. This allows creating a consumer
PipeWire stream, and receive the buffers.
Metadata cursor is always preferred, but on the lack of it, we ask the stream for
an embedded cursor (i.e. the cursor is drawn at the buffer, and OBS Studio has no
control over it.)
Window capturing is implemented as a crop operation on the buffer. Compositors
can send big buffers, and a crop rectangle, and this is used to paint a subregion
of the buffer in the scene.
The new capture is only loaded when running on EGL, since it depends on EGL to
call gs_texture_create_from_dmabuf().
[1] https://pipewire.org/
[2] https://github.com/flatpak/xdg-desktop-portal/
