[0.4.12][WIN32SS] Fix regression CORE-16721 & scrollbar bug CORE-15557

[0.4.12][WIN32SS] Fix regression CORE-16721 & scrollbar bug CORE-15557

CORE-16721 "Scrolling by pressing the scrollbar arrows does not draw the arrows as pressed"
Fixed by a patch of JIRA-user 'I_kill_bugs'. Thank you very much!

The regression was introduced by 0.4.11-dev-586-g
929a2c6
(which itself aimed to fix CORE-13986,
but not CORE-13918 as erroneously stated in commit comment)

cherry-picked from 0.4.14-dev-1167-g
8bb9434

and CORE-15557 "disabled scrollbars being clickable"
gets fixed as well due to squash of 0.4.14-dev-243-g
19fbdda