As I'm new to bugfixing, what's the story wrt to closing old bugs? What has
to happen to show that they are fixed/resolved?

PR4424 is fixed in -current. Here's a better test:

#include <stdio.h>
#include <float.h>

double d = DBL_MIN;


int
main(int argc, char *argv[])

{
        if ( d == 0.0 )
                printf("leg 1\n");
        else
                printf("leg 2, d = %f\n", d);
        return 0;
}

Andrew