Glass panes and iron bars don't connect to solid, graphically transparent blocks (slimeblock, endportalframe and spawner)

I placed a glass pane next to a slimeblock and it didn't connect, despite slimeblocks being solid blocks. I then placed a fence next to a slimeblock and it did connect.

It turns out that (stained) glass panes and iron bars don't connect to blocks that are solid, but graphically transparent. The only blocks that have this property are: Slimeblock, endportalframe and mobspawner.