enlarge-window with preserve-before non-nil

[martin rudalics]

Suppose I start with a single window called W1 (window names are
fictional)

W1

split W1 horizontally

W1 | W2

split W2 vertically

   | W2
W1 |---
   | W3

delete W1

W2
--
W3

split W2 horizontally

W2 | W4
-------
   W3

split W4 vertically

   | W4
W2 |---
   | W5
-------
   W3

and delete W2

W4
--
W5
--
W3

At this moment I can't enlarge W5 any more by "dragging its modeline
down" - the window configuration appears "frozen".  The attempt to
shrink W5 by dragging its modeline "up" has an effect similar to that of
dragging W4's modeline down, that means, W4 gets enlarged and not W3.
Likely, in this scenario siblings got confused before issuing the call
to `enlarge-window' with PRESERVE-BEFORE non-nil.