Close tab button is a circle, but...
 

... the clickable area is a square. Same as 5.1.3. I have found myself closing tabs when I didn't mean to. I click on the tab and the tip of my cursor isn't on the circle, but the tab gets closed anyway.

I always feel the cursor only selects what is at the exact tip of the pointer but you're right that the tip of the arrow can be above the close circle the close circle still selected. Other parts of the arrow are still over the circle so I don't know if this is OmniWeb's fault or the cursor being defined as an area as opposed to a point.

From my tests, the tip of the cursor is roughly 2px off. In other words, it extends 2px further than it should.

I have seen some GUI items that act as a circle, but some don't. The close/minimize... buttons for example. They only function if you click within the circle. However, when the cursor is near them they highlight.

Text selecting in OW has suffered from similar inaccuracies. I don't know if that will change in 5.5.

I agree, the cursor should only select what the exact tip of it is on.