in ui.core.js , $.ui.mouse.defaults are defined but never used. The ui components that use $.ui.mouse (draggable, resizeable, etc.) define their own cancel, delay and distance options.

This is mostly just wasted bytes (the functions work with the options undefined), but is a problem for _mouseMove, which uses _mouseDistanceMet, which requires a non-zero value for distance. A widget that does not define distance will never trigger _mouseDrag.

Quick fix: eliminate $.ui.mouse.defaults entirely and make _mouseDistanceMet work with distance === undefined by reversing the condition:

return !(Math.max(...) < this.options.distance)

(any comparision with undefined returns false)

If you use a helper for draggable, the mouseup will occur on a different element than mousedown, but the flag to prevent the click event is still set. This causes the next click to be prevented.

See ​jquery-ui-dev thead.

Updates of jQuery and jQuery UI broke the following widget code: ​http://jsfiddle.net/fyox1r56/2/ - works in jQuery 1.9.1 + jQuery 1.9.2 ​http://jsfiddle.net/fyox1r56/3/ - throws in jQuery 2.0.2 + jQuery 1.10.3

Now when widget prototype contains an object, it is cloned no matter when it's a plain object or an instance of some class with some code in its prototype chain.

This is because $.widget relies on isPlainObject functionality which was recently substantially changed in jQuery.