Prevent spurious disappearance of built-in expression filters panel

After a bit more testing following
<38b73f7100>.

Hovering the mouse cursor over the margin area would cause the panel
to spuriously disappear.

src/css/logger-ui.css

@@ -146,7 +146,6 @@ textarea {
     position: absolute;
     flex-direction: column;
     font-size: small;
-    margin-top: 0.2em;
     right: 0;
     top: 100%;
     z-index: 100;