I have the following problem in Fedora 20:
There are a lot of times that I move scroll bar very slowly. Since GTK+ detects it as a press-and-hold operation, the scroll bar changes to zoom mode. However, while I've moved the scroll bar a few lines up or down already; when it changes to zoom mode the scroll bar jumps to the initial position (or maybe not exactly the initial position; it probably converts my current movement (done before changing to zoom mode) to the equivalent zoom mode movement). This is very inconvenient and distracting. I can accept changing to zoom mode, but discarding current movement and returning back to the initial position is certainly a bug.

How to reproduce:
1. grab a scroll bar
2. move the scroll bar slowly (slow enough to trigger changing to zoom mode, but fast enough to visibly move the contents inside the scroll window before the change happens)
3. Scroll bar changes to zoom mode, but the contents of the window jumps back to nearly its initial position.