The hit/hover area where area splitting and joining can be initiated is far larger than it should be. It is so large that it interferes with other content and widgets at the corners.
For example, if you move your mouse to the top-left corner of an editor-change menu button. That button will change color as if it is "hot" and you think it will work, but the cursor will be that of the splitter and will do that action instead.
The following patch changes this from the huge area shown below on the left, to the size shown on the right. It is just as easy to split and join. It remains much larger than the visual corner, for example, and still needs much less precision than hitting the resize area. It just no longer interferes with anything else.