On fullscreen redraw the metrics of all visible paragraphs are
recreated and stored in the TextMetrics object. If the number of
paragraphs on screen does not change everything is fine because the
paragraph metrics cache is just updated. But if the number decreases
it can happen that old paragraph metrics remain in the cache. If this
happened because e.g. a paragraph was deleted, it's possible that the
cached paragraph is longer than the one which is actually at the
position now. Using the Row objects from the cached paragraphs then
can trigger accesses to positions which would have been valid in the
old paragraph, but go over the end of the paragraph which is there
now.
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18841 a592a061-630c-0410-9148-cb99ea01b6c8
