if you use a bridged plugin (e.g. renoise 32bit and a 64bit only vsti), some plugins use mouseover tooltips. There will appear if you initially opened the plugin window, click the plugin gui one time and then move the mouse over a control which has a tooltip.
Now if you click the renoise gui and click again back on the plugin gui, the tooltips won’t work anymore. You will have to close and re-open the plugin window to make the tooltips work again.
This seems to be the case only if the plugin is bridged.