dcrousso

Here's the theme I use
 

{
  "alfredtheme" : {
    "result" : {
      "textSpacing" : 4,
      "subtext" : {
        "size" : 12,
        "colorSelected" : "#FFFFFFCC",
        "font" : "System",
        "color" : "#FFFFFF66"
      },
      "shortcut" : {
        "size" : 16,
        "colorSelected" : "#FFFFFFFF",
        "font" : "System",
        "color" : "#FEFFFF66"
      },
      "backgroundSelected" : "#016BDEFF",
      "text" : {
        "size" : 18,
        "colorSelected" : "#FFFFFFFF",
        "font" : "System",
        "color" : "#FFFFFFD8"
      },
      "iconPaddingHorizontal" : 4,
      "roundness" : 0,
      "paddingVertical" : 2,
      "iconSize" : 32
    },
    "search" : {
      "backgroundSelected" : "#B2D7FFFF",
      "paddingHorizontal" : 8,
      "spacing" : 0,
      "text" : {
        "size" : 24,
        "colorSelected" : "#000000FF",
        "font" : "System Light",
        "color" : "#FFFFFFFF"
      },
      "background" : "#00000000",
      "roundness" : 0,
      "paddingVertical" : 2
    },
    "window" : {
      "color" : "#0D0D0DBF",
      "paddingHorizontal" : 0,
      "width" : 500,
      "borderPadding" : 0,
      "borderColor" : "#0000007F",
      "blur" : 20,
      "roundness" : 5,
      "paddingVertical" : 0
    },
    "credit" : "dcrousso",
    "separator" : {
      "color" : "#CBCBCBF3",
      "thickness" : 0
    },
    "scrollbar" : {
      "color" : "#6D6D6DFF",
      "thickness" : 0
    },
    "name" : "Simple Dark"
  }
}

You seem to have padding at the bottom.  I do not.  I try to keep my Alfred "window" as small as possible 😅

when the selected item is not at the bottom, the Alfred "window" has rounded corners (as desired/expected)

when the selected item is at the bottom, the Alfred "window" has square corners (not desired/expected)

I'd expect the window corner rounding to take precedence over the selected item rounding.