> If this assert is ignored then there is crash later on in my application.

I'm not reproducing this. I don't know about your application, but when the assert in tkTextBTree.c:14855 is ignored, then the assert in tkTextBTree.c:14777 triggers, and when the latter is also ignored the given test case succeeds as expected.

That said, the assert on line 14855 makes sense. It reads:
<verbatim>
    assert(SegmentIsElided(sharedTextPtr, segPtr, textPtr));
</verbatim>
which in a function named <code>TkBTreeFindStartOfElidedRange</code> makes quite some sense.