Update math macro display when entering from above/balow

This change forces metrics computation in all cases when cursor enters
a math inset from above/below, but I do not have a better idea right now.

Fixes bug #12952.

src/Cursor.cpp

@@ -2007,6 +2007,8 @@ bool Cursor::upDownInMath(bool up)
 		// try to find best position within this inset
 		if (!selection())
 			setCursor(bruteFind(*this, xo, yo));
+		// FIXME : this is actually only needed for InsetMathMacro (bug #12952).
+		screenUpdateFlags(Update::SinglePar);
 		return true;
 	}