This introduces a `default` seat operation which is used when no mouse
buttons are being held. This means there is now always a seat operation
in progress. It allows us to separate `default` code from the standard
cursor management code.
The sway_seatop_impl struct has gained callbacks `axis`, `rebase` and
`end`, and lost callbacks `finish` and `abort`. `axis` and `rebase` are
only used by the default seatop. `end` is called when a seatop is being
replaced by another one and allows the seatop to free any resources,
though no seatop currently needs to do this. `finish` is no longer
required, as each seatop can gracefully finish in their `button`
callback. And `abort` is not needed, as calling `end` would achieve the
same thing. The struct has also gained a bool named allow_set_cursor
which allows the client to set a new cursor during `default` and `down`
seatops.
Seatops would previously store which button they were started with and
stop when that button was released. This behaviour is changed so that it
only ends once all buttons are released. So you can start a drag with
$mod+left, then click and hold right, release left and it'll continue
dragging while the right button is held.
The motion callback now accepts dx and dy. Most seatops don't use this
as they store the cursor position when the seatop is started and compare
it with the current cursor position. This approach doesn't make sense
for the default seatop though, hence why dx and dy are needed.
The pressed_buttons array has been moved from the sway_cursor struct to
the default seatop's data. This is only used for the default seatop to
check bindings. The total pressed button count remains in the
sway_cursor struct though, because all the other seatops check it to
know if they should end.
The `down` seatop no longer has a `moved` property. This was used to
track if the cursor moved and to recheck focus_follows_mouse, but seems
to work without it.
The logic for focus_follows_mouse has been refactored. As part of this
I've removed the call to wlr_seat_keyboard_has_grab as we don't appear
to use keyboard grabs.
The functions for handling relative motion, absolute motion and tool
axis have been changed. Previously the handler functions were
handle_cursor_motion, handle_cursor_motion_absolute and
handle_tool_axis. The latter two both called cursor_motion_absolute.
Both handle_cursor_motion and cursor_motion_absolute did very similar
things. These are now simplified into three handlers and a single common
function called cursor_motion. All three handlers call cursor_motion. As
cursor_motion works with relative distances, the absolute and tool axis
handlers convert them to relative first.
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