> While I see that scrolling became pixel-wise indeed, it is also very
> laggy. CPU blows up to 100% and ui hangs and freezes. It works ok if
> I scroll slowly though. It happens with all my plugins and
> customizations and if I run emacs with -Q flag. My OS is Arch Linux
> and I also have HiDPI screen (it's MacBook 11,4, mid-2015).

On the previous commit <8eb6870be690128fb1cbc012c55093813c39830c>, I
revised two functions.  I fixed `pixel-scroll-down' but I broke
`pixel-scroll-up'.  I apologize for the careless commit.

With the current and broken `pixel-scroll-up', when EOB is shown on
top of the screen, emacs hangs (or goes in infinite while loop).  The
pixel-scroll-up should be reverted to commit
<1bda71ec3b11eeb4d06c3da094a3cb21bac18d5c>.

I'm sending ChangeLog and a patch relative to the current master.

* ChangeLog

Fix vertical cursor motion in pixel-scroll.el

* lisp/pixel-scroll.el (pixel-scroll-up): Do not try to move cursor down when EOB
is shown at the top.  This function is reverted to commit
1bda71ec3b11eeb4d06c3da094a3cb21bac18d5c. (bug#29737)