bug#43506: 26.1; line-height sometimes has no effect on the line height

[Markus Triska <triska@metalevel.at>]

To reproduce this issue, please start Emacs with "emacs -Q" and then
evaluate the following form:

    (progn
      (goto-char (point-max))
      (insert "\n")
      (insert (propertize "\n" 'line-height 3))
      (forward-line -1)
      (line-pixel-height))

In my case, the minibuffer then states "17". The expected result is 3,
because documentation states:

       A newline can have a ‘line-height’ text or overlay property that
    controls the total height of the display line ending in that newline.

For comparison, it works exactly as expected if I change "3" to "300" in
the snippet above, yielding "300" in the minibuffer.

If possible, could you please make line-height control the total height
of the display line also in the original example, or alternatively
consider changing the documentation to mention all relevant exceptions?

Thank you and all the best!
Markus

In GNU Emacs 26.1 (build 1, x86_64-apple-darwin15.3.0, X toolkit, Xaw scroll bars)
 of 2018-09-22 built on mt-laptop
Windowing system distributor 'The X.Org Foundation', version 11.0.11502000