struct Rect: bool IsEmpty() const { return bottom <= top || right <= left; };

[geotyper]

In struct rRect:
Rect() :
left(0),
top(0),
right(0),
bottom(0) {}
top is top coordinate, so it need be bigger than bottom coordinate, only if we not in screen coordinates, where axis y is reversal to normal axis y.
All work, but need to swap in Rect init bottom and top coordinates.
Thanks for attention.

Post remark:
I found in closed issues your comment:
Well that depends on your graphics display. In Cartesian coordinates you're absolutely correct. But in many graphics display libraries, including those I've used in developing this library, they invert the Y axis so that values increasing from 0 as you move DOWN the Y axis. In other words, rather than the origin (0,0) being bottom left, the origin is top left.

But I think it need to add some words about that on your site in Coordinate Range.
Because it not often use reverse y in 2d engines and libs.
Or add some small functions or flags that invert y Axis, but also need to now screen size minimum by axis y.

[geotyper]

And one more comment:
For all other functions of lib it is critical to swap y axis or they symmetry in respect of axis y?

[AngusJohnson]

I'm not persuaded that this is an issue, nor do I understand you followup question.

[geotyper]

In some cases this issue, because when use RectClipLines and result always zero, I was need to solve this problem, go to source of library and etc. It is more easy to read about this in article or in definition of function.
About second question:
Is reverse Y axis use in all library and need to swap y coordinates on input to Path every time use Clipper?

[AngusJohnson]

because when use RectClipLines and result always zero.

Rect<T>.IsEmpty() will always be true when (bottom <= top) || (right <= left), just as you've stated in the caption.
By convention, rectangles are generally defined such that bottom = top + height, and right = left + width.
So this is completed unrelated to whether the display's Y-axis is inverted or otherwise.