This function extends an existing selection in the following way:
If the extension point is *before* the upper boundary of the current
selection, extend the selection upwards.
If the extension point is *after* the bottom boundary of the current
selection, extend the selection downwards.
If the extension point is *inside* the current selection, shrink the
selection such that the new size is maximized. This means we move the
*closest* start/end point from in the current selection.
This enables user mappings for the left mouse button with click count
> 3
I.e. it is now possible to create custom quad-click mappings (except
we don't yet support this in footrc).
We now create a copy of the config for each client, and updates it
with the values passed from the client.
Since we're not actually cloning it (and e.g. strdup() all strings
etc) we can't call conf_destroy() to free it, but need to free just
the strings we've replaced.
This both prevents accidental resizing of the memfd, and allows the
Wayland server to optimze reads from the buffer - it no longer has to
setup SIGBUS handlers.
This lessens the burden on (primarily) the compositor, since we no
longer tear down and re-create the SHM pool when scrolling.
The SHM pool is setup once, and its size is fixed at the maximum
allowed (512MB for now, 2GB would be possible).
This also allows us to mmap() the memfd once. The exposed raw pointer
is simply an offset from the memfd mmapping.
Note that this means e.g. rouge rendering code will be able to write
outside the buffer.
Finally, only do this if the caller explicitly wants to enable
scrolling. The memfd of other buffers are sized to the requested size.
* Impose a maximum memfd size limit. In theory, this can be
2GB (wl_shm_create_pool() is the limiting factor - its size argument
is an int32_t). For now, use 256MB.
This is mainly to reduce the amount of virtual address space used by
the compositor, which keeps at least one mmapping (of the entire
memfd) around. One mmapping *per terminal window* that is.
Given that we have 128TB with 48-bit virtual addresses, we could
probably bump this to 2GB without any issues. However, 256MB should
be enough.
TODO: check how much we typically move the offset when scrolling in
a fullscreen window on a 4K monitor. 256MB may turn out to be too
small.
On 32-bit shm_scroll() is completely disabled. There simply isn't
enough address space.
* Wrapping is done by moving the offset to "the other end" of the
memfd, and copying the buffer contents to the new, wrapped offset.
The "normal" scrolling code then does the actual scrolling. This
means we'll re-instantiate all objects twice when wrapping.
