This avoids jitter when resizing splits. I didn't see any jitter before
splits but conceptually its possible. The issue is that since we're
updating the overlay DURING A RESIZE, changing the dimensions of any
part of the widget tree causes GTK warnings and a bunch of laggy
updates.
Instead, we copy the label text to a property and update it on the idle
callback along with everything else. This also provides a natural
debounce to the label.
8e073505f7