Until 9.10-HF08, the "Add content" button has a tip on the left hand side.
Since 9.10-HF09, the same button has a tip appearing right on it, whch hampers the click on the button.
This is fixed in nightly by
Please fix the tooltip appearance in 9.10 release.
(This works on Chrome only in 9.10 release; this chokes on Firefox and Edge)
Steps to reproduce;
On a new LTS 2017 instance, mp-hotfix at least on HF09 (or later HF)
Go in a workspace or folder and try to add content => If the mouse is over the " + " sign of the Add content button, the click has no effect due to the tip, which appears a second later right on the " + " sign.