There is one basic question have, about global variables. They seem to be considered bad programming style, and normally I also try to avoid them. On the other hand, the old Qt3 projects I was also working on always had a single global variable, a pointer to the main object
Global variables are a language feature, they are not evil if you use them when their use makes sense.
C++ inherited this feature from C.
In C, there are no classes, so globals where needed.
In C++, you have classes, and they have members, which means, that if you do your encapsulation right, you wont come to need globals.
But sometimes (seldom) it can come to it that a global is needed.
But how do I prevent them to:

1. Send signals back to the slider widget, avoiding an infinite cascade of events
2. Prevent the other widgets from also updating the draw area again
Have a look at the 'tracking' property.