bug#28342: 26.0.50; (OSX) Jumpy Scrolling using Trackpad

[James Nguyen]

Recipe:

emacs -Q

(setq scroll-margin 5
      scroll-step 1
      scroll-conservatively 10000
      scroll-preserve-screen-position 1
          mouse-wheel-scroll-amount '(1)
          mouse-wheel-progressive-speed nil
          mouse-wheel-follow-mouse 't)

Open another buffer (init.el for example). Use only one window.
Scroll with trackpad up and down.
Use trackpad mouse to click past the scroll margin.
Try scrolling up and down with trackpad. It will jump wildly.

Video:
https://www.dropbox.com/s/2jwb3p2sul9q36d/JumpyScrolling.m4v?dl=0

Notes: Only seems to happen when there's one window. I could not
reproduce it if I opened a second window (as shown in the video.)

Happens on both Emacs 25 and Emacs 26 (master branch).