Also confirming that MR in #6 fixes the issue.
As a data point, we are seeing this issue on 2.7 as well. Adding the lines in settings.local.php works for local but adding to environmental conditional statement in settings.php does not work on dev/staging server environments (Acquia). Running drush commands results in error "Memcache instance could not be initialized. Check memcache is running and reachable".
Reverting to 2.5 seems to be the only solve for now. Core 10.3.9
The attached patch does not build on the currently dominant approach to this issue (#54 at the moment), and allows the sub-menus to scroll (rather than expand laterally) as mentioned in the original request. It introduces a new override template with an added container div around the ul's to achieve the desired result. This required some adjustments to the CSS and Javascript, including a direct copy/tweak of some of the core toolbar module code.
This is my first contribution, so kindly let me know if I've missed any steps or approached the process incorrectly. Based on 3.5.1 running on core 10.3.6.
Was tough tracking this down, thanks for creating the issue and patch. Patch seems to work fine.