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A closed SVG path is not completely filled

In an attempt to suggest a method to round the corners of a filled rectangle, for this forum question
[Having round corners for Gantt Chart and another hover location]
(Having round corners for Gantt Chart and another hover location),
I noticed that a closed SVG path consisting in four quadratic Bezier curves and the lines connecting the end control points of the Bezier arcs
is not filled as expected https://plot.ly/~empet/14945.

Is this a bug or this is the intended behaviour?