Stack changes order after opening a new window

Window stacks do not keep a consistent order.

For instance, let's say I stacked VS Code (focused and raised) on top of Firefox.
Opening a new Firefox window will also bring the stacked Firefox window to the front.

Reproduce the reported scenario on macOS: stack a focused VS Code window above Firefox, then open a new Firefox window. Trace the window-stack ordering behavior in yabai and verify that opening another window does not move the already stacked Firefox window ahead of the focused window.

Written by the indexing model from the issue text.