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Cursor hotspot and cursor image are not updated atomically on Wayland

KDE bug report: https://bugs.kde.org/show_bug.cgi?id=488398

GTK updates the cursor in two steps:

  • set hotspot
  • attach a buffer

This can result in the cursor unexpectedly jumping if the compositor updates the hardware cursor between set hotspot and attach a buffer steps. In order to ensure that cursor updates are atomic and there are no glitches, GTK needs to update the cursor differently:

  • if the cursor needs to be updated after receiving an enter event, a new buffer should be attached to the surface before the wl_pointer.set_cursor() request
wl_surface.offset(surface, 0, 0)
wl_surface.attach(surface, buffer, 0, 0)
wl_surface.commit(surface)
wl_pointer.set_cursor(pointer, surface, hot_x, hot_y)
  • if the cursor needs to be updated after wl_pointer.set_cursor(), wl_surface.offset should be used
wl_surface.offset(surface, old_hot_x - new_hot_x, old_hot_y - new_hot_y)
wl_surface.attach(surface)
wl_surface.commit(surface)

Note that wl_surface.offset doesn't take an absolute hotspot position, instead it takes an offset relative to the old hotspot position. wl_pointer.set_cursor takes an absolute hotspot position.