In setStretchLastSection we restore a default section size
if we no longer use stretch. That size was however not
sufficient - we should restore the actual size.
Furthermore we should also always stretch the last section
(last visible index) - and not leave a section with a
huge size and stretch another.
This patch refactors stretch handling and keeps track
of the last section and its size in eg. moveSection,
swapSection, hideSection, showSection etc.
There is an auto test showing and guarding its behavior.
[ChangeLog][QtWidgets][QHeaderView] Fixed some issues
(e.g QTBUG-39010) with restoring of section size after
a section is no longer the last visible section
(in stretchLastSection mode).
Task-number: QTBUG-39010
Change-Id: Id47075b5a9dfeb250027374ecbd10eb8babbf9ef
Reviewed-by: David Faure <david.faure@kdab.com>
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