Even with noticeable delay in autocomplete popup display (bug 1287418) the result list seems to jump/being shifted just after initial display.
Please, describe the shifting you noticed. Is is duplicate of bug 1276121 or something different?
No, this doesn't seem to have anything common with bug 1276121. It was about results shifting right (due icons loading?) while populating the list, in 49b1 the effect is less visible.
I believe I experience the same, unless the window is maximised. I can return it to regular function by adjusting the left edge of the window, either moving or resizing it. Then the faulty behaviour starts again after the window has left and regained focus. This is 55.0.2 (64-bit) from the Mint repos.