Confirmed.

Unfortunately I have no coding skills, but I have recognized that each of these buttons has its own div.btn-group in Protostar, which is different to the Isis backend.

All three buttons in one div.btn-group would look like this:

(Note: I changed .btn-group > .btn + .btn to .btn-group > .btn in "template.css" to have margin-left: -1px; for the last button as well.)

Pressing "Versions" opens a modal showing such a toolbar, in case someone would like to compare.

If I only knew how and where to modify JS to access div.btn-group div#versionsModal...

In fact, it could be done via a bootstrap override in cassiopea protostar template.less

// Need .dropdown-toggle since :last-child doesn't apply given a .dropdown-menu immediately after it
.btn-group > .btn:last-child,
.btn-group > .dropdown-toggle {
  .border-top-right-radius(@baseBorderRadius);
  .border-bottom-right-radius(@baseBorderRadius);
  .border-top-left-radius(@baseBorderRadius);
  .border-bottom-left-radius(@baseBorderRadius);
}

just before
@import "template_rtl.less"; // Specific for rtl. Always load last.

then running generatecss.php to get

But it should be tested all over to know if unwanted consequences

@infograf768 Thank you for your hint. Do you think this will work for Protostar, too?

sorry this was for protostar...
(Cassiopea in J4 does not use less files, but scss)
I correct post above

@infograf768 Never mind.

I guess I better wait for a fix that may be included in one of the next minor updates. Unfortunately I do not have access to the PHP console, and it is not that important, anyway. If someone likes to work on this, I will be very pleased of course. :)

Please test #31776

Thanks a lot, folks! What a nice Christmas present! ?