In 8.6b3, the history navigation by the arrow keys and <C-n>, <C-p> is broken. This is due to the text widget reacting on virtual events <<NextLine>>, <<PrevLine>> instead of the direct key presses. The attached patch resolves the problem. It needs further checking, whether this binding change affects other keystrokes as well.
Patch for 8.6b3
Applied this minimal patch while planning a larger patch from Nijtmans. See also https://sourceforge.net/tracker/?func=detail&atid=112997&aid=3582795&group_id=12997 (tktoolkit core bug 3582795)