Only expand selection on multiple clicks with the primary mouse button

Chrome + WebKit increment the click count value for the MouseEvent even if the user switches buttons. E.g., a double click then right click will report the last event as {button: 2, detail: 3}, causing the
selection to expand to the paragraph bounds.

Now, the click count is only reported as non-zero if the current event is with the primary mouse button (left by default, or the right button if the user has switched their primary mouse button around).