I can’t resize any undocked panels with the current Linux version (5.5.0xf3Linux).
Undocked panels will be movable (though they follow roughly ~200 pixels below my mouse cursor) but there are no resize handles on their borders. I tried clicking anyway (in case the cursor just wasn’t updated) but nothing happens.
I can drag splitters between docked panels just fine.
It worked in older betas, though I’m not sure when it last worked.
I’m running Ubuntu 16.10, Gnome desktop.
I can confirm on 5.5f3, 5.5p1, 5.6b3 too.
There’s no resize handle, the cursor is still normal arrow, and I’m not able to resize by dragging the any pixels of edges.