The docs simply states
RoundedRectangleBorder
A rectangular border with rounded corners.
Typically used with ShapeDecoration to draw a box with a rounded rectangle.
This shape can interpolate to and from CircleBorder.
BorderRadius.circular
Creates a border radius where all radii are [Radius.circular(radius)].
What does this mean? If my button is 50 logical pixels (25 radius), and I set the radius to 20 should it then clip the corners outside of the 20 logical pixel radius? If I set it to 30 the whole button would be within the circular radius, so nothing should be clipped. This is not the case.
Everything >=30 seems to clip the corners to a 45 degree arc, resulting in a complete half circle on each short side of the button.
Can anyone explain this value and how to use it?
The radius is telling you how far from the corner that the arc is created. So if you want just slightly rounded corners you'd use a smaller value. A DecoratedBox that's 80pt x 30pt might take a 7pt circular radius. A circle who's radius extends from both the top and adjacent side by 7pts. If you applied a circular radius that was larger than the shortest available side that's where you run into the half circle. For this, a radius of 15pt and larger would create that effect.
If you're looking for a box that might fractionally decide it's rounded corners for you then you could easily create a class that wrapped a DecoratedBox inside of a LayoutBuilder to figure out the shortest side and determine a fractional radius based off of that length.