Stacking window manager

A stacking window manager (also called floating window manager) is a window manager that draws all windows in a specific order, allowing them to overlap, using a technique called painter's algorithm. All window managers that allow the overlapping of windows but are not compositing window managers are considered stacking window managers, although it is possible that not all use exactly the same methods. Other window managers that are not considered stacking window managers are those that do not allow the overlapping of windows, which are called tiling window managers.

Stacking window managers allow windows to overlap by drawing them one at a time. Stacking, or repainting (in reference to painter's algorithm) refers to the rendering of each window as an image, painted directly over the desktop, and over any other windows that might already have been drawn, effectively erasing the areas that are covered. The process usually starts with the desktop, and proceeds by drawing each window and any child windows from back to front, until finally the foreground window is drawn.

The order in which windows are to be stacked is called their z-order.

Stacking is a relatively slow process, requiring the redrawing of every window one-by-one, from the rear-most and outer-most to the front most and inner-most. Many stacking window managers don't always redraw background windows. Others can detect when a redraw of all windows is required, as some applications request stacking when their output has changed. Re-stacking is usually done through a function call to the window manager, which selectively redraws windows as needed. For example, if a background window is brought to the front, only that window should need to be redrawn.

A well-known disadvantage of stacking is that when windows are painted over each other, they actually end up erasing the previous contents of whatever part of the screen they are covering. Those windows must be redrawn when they are brought to the foreground, or when visible parts of them change. When a window has changed or when its position on the screen has changed, the window manager will detect this and may re-stack all windows, requiring that each window redraw itself, and pass its new appearance along to the window manager before it is drawn. When an application stops responding, it may be unable to redraw itself, which sometimes causes the area within the window frame to retain images of other windows when it is brought to the foreground. This problem is commonly seen on Windows XP and earlier, as well as some X window managers.

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