X Windows "window swallowing" is the process of program 1 capturing the main window of program 2 to display it inside of program 1's main window. For example, Firefox using an external program to display a PDF in a tab. Firefox's built-in javascript PDF viewer leaves a lot to be desired. Slow rendering or no rendering at all, memory hog, with the download button not working when it doesn't render.
The following description is derived from my experience getting MozPlugger to work in Firefox to display PDF files using Evince.
Continue reading X Windows - Window Swallowing