Combining the trick exposed in http://lzone.de/Liferea-Trick-7-Force-Read-Full-Posts with the trick of middle-clicking a feed item to mark it as read (as also mentioned in http://lzone.de/Liferea-Trick-1-Middle-Mouse-Button ) will lead to a bug where Liferea marks two items as read instead of one.
1- have a folder where you have a "normal" feed that we will call "A" (where you read only the syndicated contents in liferea) and one feed that you have configured to load the full feed (thus showing as an embedded webkit view instead of a liferea-formatted view) that we will call "B".
2- Configure liferea to show the unread feed items when clicking a folder. Click the folder to display them.
Sometimes there will be some unexpected behavior where, instead of toggling only the item under the mouse pointer (regardless of selection etc.), it will mark both items as read, and sometimes take them out of the listview completely (in folder view).