The layers library and Intuition maintain a damage region for each window,
which is the part of the window whose imagery is in need of repair, or
refreshing.  Several things can add areas of the window to the damage
region:

  * Revealing an obscured part of a Simple Refresh window adds that area
    to the damage region

  * Sizing a Simple or Smart Refresh window bigger along either axis adds
    the new area to the damage region

  * Resizing a Simple or Smart Refresh window (smaller or bigger) adds
    the old and new border areas, and the areas occupied by certain
    gadgets (those whose position or size depend on window size) to the
    damage region.