The feature is added by bug 1278574. It is more user-friendly to show the buffering icon when MDSM doesn't have the resource to continue decoding. Here are some implementation details: 1. MediaFormatReader::RequestVideoData() rejects the promise with WAITING_FOR_DATA when it is suspended. 2. MDSM calls MediaDecoderReader::WaitForData() and enters buffering state. 3. MediaFormatReader resolves the waiting promise when it is resumed. We will be able to remove the new code path added to MDSM by bug 1278574. MDSM will run the code path (the same one as data gap encountered in MSE) that is well-tested.

It seems easier to enter/exit dormant when the reader is suspended/resumed. It also makes sense to employ dormant mechanism to handle reader suspend/resume because ReleaseMediaResources() is called when suspending a reader. We need to call ReadMetadata() to recover the reader which is exactly what the dormant mechanism does. https://hg.mozilla.org/mozilla-central/file/331c4166a3a2df2d3a037107addef5d85cdc31b5/dom/media/MediaDecoderReader.cpp#l164

Oops! We already have bug 1292453.