Version: 51
OS: 10.11

What steps will reproduce the problem?
(1) Add at least one extension to the browser
(2) Position the mouse near/just past the right edge of the location bar, so that it turns into a horizontal resize cursor
(3) Drag the location bar's right edge to the right so that it becomes larger

Once the mouse drag moves over the left-most extension, that extension button highlights, as if you had moved the mouse over the button and were about to click it.