MXR extracts metadata about files to display in the "description" column of directory listings to provide more information about a file. This is useful especially for test files to figure out which file one should add a test case to since people try to keep file names shorter and there was a bad habit of file names like "test_bug798843_pref.html". That name isn't useful for skimming initially but the description shown in MXR helps a lot: "Test that SVG glyph context-* values can be pref'ed off" Off hand I think that MXR extracts from <title> and from doc-comments.
Useful information from bug 1177125: "For directories that contain a file called README/README.txt the description is taken from the first non-empty line in that file. For files that begin with a Doxygen style comment the description is taken from the first line of that comment."
Digging deeper, it looks like MXR's implementation is governed by two routines. fdescexpand (https://mxr.mozilla.org/webtools-central/source/mxr/Local.pm#27) pulls descriptions out of files. descexpand (https://mxr.mozilla.org/webtools-central/source/mxr/Local.pm#251) describes folders in listings.
Created attachment 8761677 [details] [review] pull #561 https://github.com/mozilla/dxr/pull/561
Commit pushed to master at https://github.com/mozilla/dxr https://github.com/mozilla/dxr/commit/adc8428520efe88e4385f5a426fb3c272916387b Add a descriptor plugin that emulates MXR's descriptor column. Also add some framework for plugins to supply needles on folders and adding headers to the browsing view. Fixes bug 1157968.