Dear Xara...

They aren't.

Again, I have a simple task - make a random triangle (one I actually want) and round its corners.

Triangles 1, 2, 5 & 6 are equilateral triangles, all with rounding. They are identical in form and function.
Stretch any and the rounding is equally skewed. I expect circular corners.
On no, I am wrong. Triangles 5 & 6 are difference - you rotated them 90o clockwise.
If you experiment with Star 1, you can replace all of the above with it and get curves for free.

Triangles 3 & 4 (and 7 & 8). Identical. Now none has rounding.
You can only adjust the Height or separately the Width.
Gosh, you rotated 7 & 8 too.

Now we get mixing with right-angled Triangles 9 & 11 (with 10 & 12 (out of sequence but rotated)).
Same limited controls as the last set.

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Three designs:
  1. Equilateral triangle with vertex rounding. Sorry a Shape that can have 3 to 30 vertices.
  2. Isosceles triangle with H & W controls.
  3. Right-angled triangle with H & W controls.

No Acute triangle. I discount skewing.
No Scalene triangle. I discount skewing.
No Obtuse triangle.

Sorry I should have mentioned Trapezoids 1, 2 & 3. You can almost make all of the above with these.
Can you see the problem?

All of these need a Smart Shape with the Controls on the vertices.
I could forego rounding over gaining a generic triangle-maker.
I can manage from there.

I would also be pleased if each vertex could be individually rounded.
If I skew a triangle, I would expect a reset back to the original form.

What I would be really amazed with is some of your dragging blue controls were actually aware of the Xara canvas and other objects, grids or guides.

Acorn