> Expected result:
>
> The regexp-builder window takes 4 lines, just as when
> temp-buffer-resize-mode is not enabled.
>
> Workaround:
>
> Calling M-x fit-window-to-buffer after M-x regexp-builder resizes the
> window successfully.

Before 57354bc6 it was hard-coded to 4, so a possible fix would be

              (window-height . ,(+ (if header-line-format 1 0)
                                   (if tab-line-format 1 0)
                                   4))

But a better fix is this: