JQuery width() height() always get stage size instead of div size?

I have an Edge project with nested Symbols. In a 3rd level symbol instance, I log it's real size like this.

var stageWidth = $(window).width();

var w = sym.getSymbolElement().width();

console.log(w,stageWidth);

When it's in the creationComplete event handler, it shows the right size.

When it's in the mousemove event handler, it always shows that div size == stage size.

I have tried hours not to find the solution.

What is this problem?