Made the change below just now (not yet pushed to GH), in attempt to fix this issue (not yet tested on Safari):
MS sign-in panel retrieves the auth-link at time of panel open, rather than time of sign-in-button click. By doing this, the sign-in button can launch the sign-in popup immediately, rather than after a slight delay. (in attempt to fix popup-blocked issue on safari)
After making the attempted fix (in previous comment), M-A did a quick test on a Safari browser, and did not hit the issue. (granted, the issue was unreliable there)
Made the change below just now (not yet pushed to GH), in attempt to fix this issue (not yet tested on Safari):