Xfce4-panel starting from version 4.9.0 supports multi-row layout and two vertical modes.
Currently xfce4-places-plugin doesn't follow these conventions (in fact it doesn't build at all due to the exo version issue #5754).
Created attachment 4288
A patch implementing these features
The patch contains two commits - the first one is just a quick fix for the exo version issue. Is probably better to use another patch for it.
The second commit is the actual patch adding support for 4.9+ features. It's compiled optionally so the plugin should still work with earlier versions of the panel.
Fixed in release 1.3.0 with further UI fixes in 1.4.0.
Mass-reassign all bugs from florian@ to goodies-dev@, thanks for the maintenance work! (and sorry for the bugmail spam..)
Fixed as indicated in the comments.