bug#29737: 27.0.50; pixel-scroll-mode is laggy

[Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>]

>>> 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.
>
> I'd like to fix this on the release branch, not on master.  Is the
> patch you sent good to go to the release branch?  Does it solve the
> display lags mentioned in the bug report?

Yes, the patch I sent is good to go to the release branch.

I'm not 100% sure what 'the display lag' meant.  I think the bug is
the source of 'the display lag'.

After the patch, at least I tested with MacBook and do not recognize
'the display lag'.