Identify the ListBox row at a point

Add a public ListBox.RowAtPoint(x, y) method that returns the zero-based index of the visible row currently displayed under the given ListBox-relative logical coordinates, or -1 when the point is not over a row. It is intended for use in ListBox mouse handlers, where it lets a MouseDown handler identify the row that was pressed so the value can be stored for later use by SelectionChanged.

Today a developer who needs to know which row the pointer is over during MouseDown is forced to reconstruct the row from the pointer position, and the natural workaround is to subtract a hard-coded header height (for example 32 points) plus row heights. That is fragile because the header height is theme-dependent. A native hit-testing API solves this correctly: the points received by the inherited MouseDown, MouseUp, MouseMove and MouseDrag events are already ListBox-relative regardless of window, container or scroll position, so RowAtPoint can be fed those coordinates directly and account for headers, scrolling, borders and nesting itself.

For pointer-driven ListBox selection, the Objo MouseDown handler is delivered before SelectionChanged, so the pattern of storing pressedRow = Me.RowAtPoint(x, y) in MouseDown and reading it in SelectionChanged works. This ordering guarantee applies to Objo callbacks; it does not necessarily mean SelectedRowIndex still contains the old selection during MouseDown, because the native control may already have updated its selection internally.