Unclosed paths have some issues, when taking part in intersections,
so stroker seems to be only solution for now.

Draw highlight outline, for if outline is empty (Qt.NoPen), highlight entire background.
TODO: Do we need full bg/fg/outline hover-highlighting support?

Previous square cap style caused it to be drawen slightly out of end
point, and that part sometimes appeared above port, sometimes not
(usually if you select one port, connection covers port on another
side).

Use ready values for right and left sides, also making code slightly
better readable (probably subjective though).

Group box may get non-integer coordinates if, e.g. moved with scaled viewport.
When it happens, previously sharp lines between to draw between pixels,
while previously blurred lines may reversly get a sharp look.

When in scaled state, graphics still may be badly-antialiased though.

Text in port is aligned by center.
In group it is centered in header rect.
When icon is supported, text is aligned vertically with it,
and centered between icon and right group edge.

There problem with intersection, causing absent outline from one side, if not using that magical value 0.001 (any non-zero)

Clipping is enabled automatically, when clip shape is set.

Polygon was not enclosed automatically, when added to path.

What about to deliver to theming, wether ports cover group edge or not?

When drawen outside, free space between ports is busy.
But when inside, outline is overlaped with text, so themes need bigger height to fix that.

Necessary for classic theme. Now it will look like in latest patchage/ingen series.

Smaller value is guessed by distance between bottom box side and ports.

The drawRect() doesn't have variant, using qreal values, but it accepts QRectF. Even for integer coordinates it uses subpixel positioning, thus always placing them exactly between pixels.