W is the handle of the window obtained using %hw.
Sometimes it is useful to freeze the visible contents of a window whilst a
number of changes are made. This is particularly true when the window contains
a frame with child sub-windows. By freezing a window, performing a sequence of
changes, and unfreezing the window, the contents of the window will appear
to change only once.
This routine covers the window in question with another window containing a
copy of the contents of initial window. Therefore it is important that the
window is not moved whilst frozen and it should only be frozen
for a very short period of time.