In usability testing of the Express App business object editor which uses the tree view in the XA project manager, we found that users did not understand the dot / bullet that appears in Swing's Java Look and Feel (Metal) Tree View. The bullet is used to expand/contract a branch. To quote from the usability test report:

"Bullets in the project manager were "not intuitive" and did not suggest an
outline view. Most did not think that clicking on the bullet would expand
the outline. One participant wanted to see a "+" and "-" or open and closed
books, which is what he/she is more familiar with."

The usability test gave the problem a rating of XXX, which is the lowest rating on a scale of 1-3.