These objects are exposed by the MozDOMAfterPaint event. Each one represents
a request to repaint a rectangle that was generated by the browser.
The client rect where invalidation was triggered.
The reason for the request, as a string. If an empty string, then we don’t know
the reason (this is common). Reasons include “scroll repaint”, meaning that we
needed to repaint the rectangle due to scrolling, and “scroll copy”, meaning
that we updated the rectangle due to scrolling but instead of painting
manually, we were able to do a copy from another area of the screen.