Window managers are another consideration when programming with overlays. Depending on the window manager, border decoration may or may not be created to exist in the overlay planes. If the borders are not in the overlay planes, expose events for underlying normal plane windows will still be generated when the overlay window is moved or unmapped due to the damage caused by the window manager borders. The chief advantage of creating overlay windows is negated. But if the window manager does create the borders in the overlay planes, the colormap used by the window manager will be different from the colormap for the client window nearly guaranteeing colormap flashing.
The best advice is to create toplevel overlay plane windows enabling override-redirect or without any window manager decoration. Since most uses of toplevel overlays are for transient windows, this advice is generally easy to follow.
In summary, due to the fact that overlays are generally shallower than the normal planes visuals and not the default visual, there are a number of considerations to take into account when creating overlay windows. Efforts to avoid exposes or colormap flashing also means window manager decoration should generally be avoided for overlay windows. Because overlays are used for transient windows mostly, these limitations are generally acceptable if client writers are aware of them.