I recently figured out how to add separators to columns in the tree widget by creating a fake attribute (see https://www.awareim.com/forum/viewtopic.php?f=1&t=10502). This presents the need to hide a separator when the preceding attribute is empty as shown in the example. Fortunately, there is a feature for that under Column Settings. Unfortunately, that feature does not seem to be working, at least not for tree widgets. I tried: