Or, what you are building requires somewhat complex components, but you'd rather focus on something else? Many frameworks have their own component libraries.

I'm not talking simple tooltips, I'm talking about real components. Sencha's tree.Panel comes first to my mind: http://docs.sencha.com/extjs/5.0/5.0.1-apidocs/#!/api/Ext.tr...

It has extremely powerful feature set (based on the other features of the framework) out of the box and you can very fast switch your focus on other things besides re-implementing the wheel.

I simply haven't found some of these things as simple libraries. They nearly always come within complete frameworks, because they have had to implement a lot of stuff that really just requires other features from frameworks. And, no, I'd rather not spend a year implementing that feature set.