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What is an abandoned document?
Abandoned documents are effectively leaked: they are Document objects which have never been destroyed, and persist after loading about:blank and clearing caches. This is probably because there's some object that is holding a reference to the Document object, possibly in the GC heap, or via a retain cycle (often involving Nodes in the document).
All documents are referenced by Document::allDocumentsMap(), so they are not leaked in the sense that the 'leaks' tool doesn't show them.
Why are layout test results showing me a list of abandoned documents?
Because leaked documents usually entrain a lot of other objects, which can use lots of memory (e.g. via entries in the memory cache). Leaking documents is bad because it will cause ever-increasing memory use as the user browses. See https://bugs.webkit.org/show_bug.cgi?id=186214
What should I do if see a new case of document abandonment?
If you made a code change that is causing a test to newly show that a document is leaked, it probably means you have a coding bug that is triggered a leak or (more likely) a reference cycle. You need to resolve this before committing.