The Add first child thread is initiated when it is requested that a folder, opened in Tree view, be populated. Because in the open Tree view, every visible folder containing subfolders must have a plus (+) sign next to it, each visible folder must be populate one level deep to see if it contains at least one subfolder. Because there can be many such folders, this processing is done on a thread other than the main User-interface thread.

The Add first child thread is also created when a folder is added to or removed from:

so that its plus sign can be modified appropriately. This thread uses a round robin queue to fairly service folders that need processing. This is so that if several folders are opened in Tree view, visible subfolders will be processed in turn, rather than one view being handled at a time. When this queue is full, _wpAddFirstChild is invoked synchronously. The Add first child thread creates a PM message queue.

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