+1 -1 = 0, more input is need to start the engine. :-)
Actually, creating a patch is easier than putting it through, at least in this case. Index: src/mathed/InsetMathHull.cpp =================================================================== --- src/mathed/InsetMathHull.cpp (revision 18740) +++ src/mathed/InsetMathHull.cpp (working copy) @@ -335,7 +335,7 @@ void InsetMathHull::draw(PainterInfo & pi, int x, int y) const { - use_preview_ = previewState(pi.base.bv); + use_preview_ = previewState(pi.base.bv) && !mouseHovered(); // background of mathed under focus is not painted because // selection at the top level of nested inset is difficult to handle. Cheers, Bo