Detection was already mentioned here a few times. Personally I don't think it would be a sound thing to make StrokeIt detect internally where the cursor was at the point of the gesture start. There are some tricks in Lua that can detect child classes after the gesture but restricting where it should not even start a gesture would cause confusion. There are many different people with many different habits and some of them would not like StrokeIt to be disabled in a textbox or over a scrollbar.
Your suggestion is a havoc, particularly in a textbox. Imagine that you select the text and then you face the fact that you cannot use gesture to copy it. Of course, you could move the cursor out of the textbox and the copy function would still work but that can be annoying and require some precision if the textbox is quite large.
IMHO LMB is not a good choice for gesture trigger at all, because many of the drag and drag-n-drop features are initiated with LMB. If you still want to stick to it (and why should you not, it's your personal preference) you could decrease gesture timeout to an acceptable value, a trade-off between speed and lag.
Even so (if it is so important) it could be done with a config/setting file that contains subclasses not to initiate a gesture but that would require StrokeIt to detect the gesture's starting point class. I doubt the gain is worth the pain, but who knows...