Row gap is zero on purpose. The line joining a parent to its children runs down the column gap and has to be unbroken, and it is assembled from a segment contributed by each child - so any row gap would show as a break in it. Vertical spacing comes from the buttons' own margins instead (see .treeCell in extra.css).

Space between one root hierarchy and the next, where a project has more than one. The spacing is a CSS class rather than an inline style so it can be dropped from the last one - a margin under the last root shows as a band of panel below the tree.

Width of the column gap, which is where the connectors are drawn. Passed to the CSS as a variable so the lines and the gap cannot drift apart.