If the Replace box is not enabled/visible on the Find Bar, and you click on the right arrow button to enable the Search features, an immediate click on the Incremental Search button often ends up in the Replace button toggling to the depressed state and the Incremental Search button not staying toggled ON. A subsequent click on the Incremental Search button correctly toggles it on/depressed.
Also, after the first click of the Incremental Search, click on other buttons can end up toggling other buttons.
So their appears to be a bug in the logic of the code.
Also, enabling/disabling the Replace button causes the other toggle buttons to toggle OFF, losing their settings.