This function is supposed to be called in
implementations for widgets that support multiple windows.
cr must be untransformed from invoking of the draw function.
This function will return
true if the contents of the given
window are supposed to be drawn and
false otherwise. Note
that when the drawing was not initiated by the windowing
system this function will return
true for all windows, so
you need to draw the bottommost window first. Also, do not
use “else if” statements to check which window should be drawn.
a cairo context
the window to check.
window may not be an input-only
window should be drawn