Web ListBox headers show no separator between columns when GridLinesVisible is False

A Web ListBox with more than one column draws no separator between the header columns unless GridLinesVisible is True. With the property at its default of False, the header is one continuous band: there is no visual indication of where one column ends and the next begins, and no cue for where to drag to resize a column — the resize handle is an invisible strip at the right edge of each header cell, hinted at only by the col-resize cursor on hover.

Expected behaviour: when headers are shown and there is more than one column, the header always shows a separator between the columns. GridLinesVisible should govern only the grid lines in the cell area, not the header separators. This matches the Desktop ListBox, whose column headers always show separators between columns regardless of GridLinesVisible.

The missing separator appears both in a running web app in the browser and in the Web page designer preview.