bug#20847: [display engine] 25.0.50; company-mode popup makes point jump to an entirely different location

[Dmitry Gutov <dgutov@yandex.ru>]

1. Install company (from GNU ELPA, for instance).

2. Paste the contents of
https://gist.githubusercontent.com/sooheon/97a62f433897b52da3d1/raw/a42af658ec3ccd11a4faa7e2581f7413687b1811/gistfile1.txt
into the scratch buffer.

3. Look for the line with "hello halleo helo" at the end of the first
paragraph, and delete or add a few characters before it, to make sure
that the last "h" is two columns away from the right window border. So
as when you type "el" after it, the cursor is displayed in the margin.

4. M-x company-mode

5. (setq company-backends '(company-dabbrev))

6. Go to after the last "h", type "el", wait 0.3 seconds, see the
completion popup displayed, while the cursor is displayed in the margin.

Problem 1: even though the overlay's `cursor' property places it at the
same line as where "hel" ends, the cursor is rendered in the margin two
lines below.

Problem 2:

7. Type "l", see it wrapped to the next line.

8. Type backspace. See the cursor move to the second paragraph.

9. Continue backspacing. See the completion popup disappear, and the text
being deleted in the second paragraph.

Originally reported at
https://github.com/company-mode/company-mode/issues/362.