fix popup positioning for plasma-style popups

--consider the panel background roundness and position
positions only related to that roundness. This way the
plasma style popups that feel part of the panel background
are positioned properly always and they do not touch
in empty areas.

containment/package/contents/ui/BindingsExternal.qml

@@ -232,8 +232,8 @@ Item {
             if (root.behaveAsPlasmaPanel
                     || !LatteCore.WindowSystem.compositingActive
                     || (!parabolic.isEnabled && root.userShowPanelBackground && plasmoid.configuration.panelSize===100)) {
-                var paddingtail = (root.isHorizontal ? background.paddings.left : background.paddings.top) + background.tailRoundnessMargin;
-                var paddinghead = (root.isHorizontal ? background.paddings.right : background.paddings.bottom) + background.headRoundnessMargin;
+                var paddingtail = background.tailRoundness + background.tailRoundnessMargin;
+                var paddinghead = background.headRoundness + background.headRoundnessMargin;
 
                 if (root.isHorizontal) {
                     return Qt.rect(latteView.localGeometry.x + paddingtail,

containment/package/contents/ui/background/MultiLayered.qml

@@ -190,6 +190,30 @@ BackgroundProperties{
         return Math.max(background.length + totals.shadowsLength, totals.paddingsLength + totals.shadowsLength)
     }
 
+    readonly property int tailRoundness: {
+        if ((root.isHorizontal && hasLeftBorder) || (!root.isHorizontal && hasTopBorder)) {
+            var customAppliedRadius = customRadiusIsEnabled ? customRadius : 0;
+            var themePadding = themeExtendedBackground ? (root.isHorizontal ? themeExtendedBackground.paddingLeft : themeExtendedBackground.paddingTop) : 0;
+            var solidBackgroundPadding = root.isHorizontal ? solidBackground.margins.left : solidBackground.margins.top;
+            var expected = customRadiusIsEnabled ? customAppliedRadius : Math.max(themePadding, solidBackgroundPadding);
+            return Math.max(0, expected - metrics.margin.length);
+        }
+
+        return 0;
+    }
+
+    readonly property int headRoundness: {
+        if ((root.isHorizontal && hasRightBorder) || (!root.isHorizontal && hasBottomBorder)) {
+            var customAppliedRadius = customRadiusIsEnabled ? customRadius : 0;
+            var themePadding = themeExtendedBackground ? (root.isHorizontal ? themeExtendedBackground.paddingRight : themeExtendedBackground.paddingBottom) : 0;
+            var solidBackgroundPadding = root.isHorizontal ? solidBackground.margins.right : solidBackground.margins.bottom;
+            var expected = customRadiusIsEnabled ? customAppliedRadius : Math.max(themePadding, solidBackgroundPadding);
+            return Math.max(0, expected - metrics.margin.length);
+        }
+
+        return 0;
+    }
+
     readonly property int tailRoundnessMargin: {
         //! used from contents geometry in order to remove any roundness sectors, e.g. for popups placement
         if (root.isHorizontal) {