Index tree context menu used in "free area" should operate on toplevel namespace

[zsquareplusc]

Context menu in index tree does not operate on "toplevel" when opened in free area (no tree item under the mouse pointer), instead it operates on the last selected entry.

in more words:

Situation:
- Notebook with sub-pages
- sub page is selected
- context menu in index (sidebar) is used with "New page..", the mouse pointer was in the free area under all other entries in the tree, i.e. not pointing at any entry.

Expectation:
- the page is created where the mouse points to
- it points to no page, so expectation is "toplevel"

What actually happens:
- the page is created in parallel to the currently selected page.

It probably functions as expected with one minor issue. What I expected was that with right click, any entry below the mouse is implicitly selected and operated on (it does this), but if there is no entry below the pointer at that moment, it should work on top level.

[Jaap Karssenberg (jaap.karssenberg)]

Tried fixing, but turns out x,y positions outside list get contributed to the last item in the list :(