This is a remark to help others, should they encounter the same problem....

On a horizontal menu in Harmony, I had a 'parent' item (as a second last item on the righthand side) on which the sub-menu consisted of 2 columns.
Normally (without any custom CSS changes) the submenu unfolds down and to the right of the 'parent' item. It is (normally) only the very last menu-item (last to the right...) that unfolds its submenu to down and to the left.

But in my case the second last item had a larger width submenu (2 columns). The submenu therefore unfolded beyond the rightside of the page limit. It resulted in showing a horizontal scrollbar on the webpage and if scrolled to the right, a blank edge showed up to the right of the page, which is not what I wanted.

Here is my solution....
I can make the second last menu-item also unfold its submenu to down and left by using this CSS rule:

.hornav > ul > li:nth-last-of-type(2n) > ul {

    left: auto !important;

}

This rule defines to (just like the li:last-of-type ) to use 'left' unfolding instead of the standard 'right' unfolding.

Hope it helps anyone...