Recent folding issues

[Jack Kamm]

I started noticing a couple issues with folding, after updating my
org-mode in recent months:

1. Inserting text below or above a folded headline will cause it to
unfold. I am not sure if this is an intentional change, but I find the
new behavior confusing -- usually I am trying to enter a new headline,
and the unfolding causes me to lose my context and forget what level I
wanted my new headline to be.

2. When folded, I frequently found multiple headlines to be displayed on
the same line, like so:

* Headline 1...
* Headline 2...* Headline 3...
* Headline 4

Hitting Shift-Tab a few times (org-global-cycle) usually fixed the
problem.

After reading ORG-NEWS, I found out about org-fold-core-style, and have
set it to 'overlays, which solved both of my problems.

However, this makes me wonder whether it should be the default
value. The help for it says:

> Can be either ‘text-properties’ or ‘overlays’.
> The former is faster on large files, while the latter is generally
> less error-prone.

Since the latter is less error-prone, shouldn't it be the default? And
then a user can switch the value if they have really large files and
need better performance. It's great that performance is being improved
for large files, but I'm not sure this is ready to be the default yet.