To avoid the top shadow, add a vertical offset and adjust other parameters accordingly. Also, set the propagation distance to 0 or more, and you will cover your horizontal border.
Start with this:
box-shadow: 0px 10px 10px 0px rgba(0, 0, 0, 0.49)
If you do not get a full horizontal border, increase the fourth value a bit until it looks good. Adjust your vertical offset if necessary.
It would be helpful to see your html / css for the actual window.
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