The “metadata-container” (the entire section with the Properties) has some margins and paddings by default. I’m guessing that’s what you are seeing.
You could try this CSS snippet to see if it helps any. You can adjust the values or comment out the margin or padding if the 0px is too tight.
.metadata-container.is-collapsed {
margin-bottom: 0px;
padding-bottom: 0px;
}