Its for now an temporaly solution untill they make a fix.
My sollution is just when the menu-bar is on the left side.
How to fix it for the rightside, you must try it in the same block code.
search in style.css for this block:
/* Right Content */
.left-sidebar #primary {
float: right;
margin: 0 0 0 -26.4%;
width: 100%; /* change the 100% into 90% */
}
And change the 100% into 90%
Here the same faillure.
It looks like all pages in sidebar sjablone just like they are in standard template and overwrire the sidebar-menu.
The faillure came with the new update from WordPress 4.5
