wlcursorfix

This is a small LD_PRELOAD shim designed to add support for cursor-shape-v1 to software that does not yet support it. It is designed specifically to address the visual problems caused by applications that exhibit different behavior for scaling or cursor loading. When using a compositor that supports cursor-shape-v1, this shim should fix most applications to have uniform cursor behavior.

Note that this does not improve memory usage or anything like that; it should have a (hopefully negligable) performance hit, but it is specifically designed to address the visual issues, not the additional memory usage caused by having applications load their own cursors. This shim does not prevent applications from loading their own cursors, it just tries to replace set_cursor calls with set_shape calls.

How it works

This library only works if the application either uses libwayland-cursor, or GTK4.

In both cases, we need to hook the wl_registry listener, so we do this by overriding wl_proxy_add_listener. We also need to hook certain Wayland calls, which we do by overriding wl_proxy_marshal_array_flags - this is what allows us to mask and override calls. This gives us the primitives we need to grab the cursor-shape-v1 extension and intercept set_cursor calls to replace them with set_shape calls, but from wl_proxy_marshal_array_flags we only will know what wl_buffer is being set - we need to figure out what cursor shape corresponds to each wl_buffer object.

For libwayland-cursor applications, it hooks wl_cursor_theme_get_cursor calls. When wl_cursor_theme_get_cursor is called, all of the buffers are added to a hash table mapping buffers to shapes. This is all that is needed to support most applications, and this is expected to be relatively stable. We do rely on a couple of libwayland internals, but they are probably not going to break.

GTK4 is a little more complicated. It doesn't use libwayland-cursor, but instead it has its own vendored copy that loads cursors on-demand and handles multiple cursor sizes within a single wl_cursor_theme. Thus, for GTK4, we maintain a mapping of buffers to shapes as in the previous method, but when GTK4 is loaded, if a buffer isn't in the map, we grab the current GTK cursor theme off of the GdkWaylandDisplay, using the private _gdk_wayland_display_get_cursor_theme. Because that symbol is private, we get it by manually traversing the symbol table of the GTK4 binary in memory. To actually use it, we need the GdkWaylandDisplay. We get this, however, when the wl_registry listener is registered, as GdkWaylandDisplay will register the wl_registry with the userdata set to the GdkWaylandDisplay. So, after we detect gtk_init, we wait for the next wl_registry listener and save the userdata as the GdkWaylandDisplay device. And that's all there is to it: now, when encountering a new wl_buffer, we can search the cursor theme and record what shape it corresponds to. (For performance, we also record when a shape is not found.)

The GTK4 code is definitely more fragile, but nonetheless a lot of the details it relies on have not changed in years, so it probably won't break overnight.

How to use

Note that this is pretty ugly software and it may cause any number of unknown side-effects; if you find any, feel free to report them, although I can't guarantee I can fix them. If you want to give it a shot, it requires:

meson
ninja
pkg-config
libwayland
wayland-protocols
GLib
A C compiler

To build it, you can do something like:

mkdir .build
cd .build
meson setup ..
ninja

And then you can try running it with a Wayland application:

LD_PRELOAD=$PWD/libgtkcursorshape.so nautilus

If you experience problems, you can set G_MESSAGES_DEBUG=wlcursorfix to get verbose debug messages; this may help narrow down where things are going wrong.

jchv/wlcursorfix

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