[applets/userswitcher] Fix some sizing issues in full representation

Wrong sizing and constraints sometimes caused the popup to be too small.

applets/userswitcher/package/contents/ui/ListDelegate.qml

@@ -29,10 +29,11 @@ Item {
 
     property Item highlight
 
+    // sizing: top-down explicit width, bottom-up implicit height
+    implicitHeight: row.implicitHeight + 2 * PlasmaCore.Units.smallSpacing
+    implicitWidth: row.implicitWidth
     Layout.fillWidth: true
 
-    height: row.height + 2 * PlasmaCore.Units.smallSpacing
-
     MouseArea {
         id: area
         anchors.fill: parent

applets/userswitcher/package/contents/ui/main.qml

@@ -107,7 +107,11 @@ Item {
         id: fullRoot
 
         Layout.preferredWidth: PlasmaCore.Units.gridUnit * 12
-        Layout.preferredHeight: Math.min(Screen.height * 0.5, column.contentHeight)
+        Layout.preferredHeight: column.implicitContentHeight
+        Layout.minimumWidth: Layout.preferredWidth
+        Layout.minimumHeight: Layout.preferredHeight
+        Layout.maximumWidth: Layout.preferredWidth
+        Layout.maximumHeight: Screen.height / 2
 
         PlasmaCore.DataSource {
             id: pmEngine
@@ -135,13 +139,19 @@ Item {
             id: column
 
             // there doesn't seem a more sensible way of getting this due to the expanding ListView
-            readonly property int contentHeight: currentUserItem.height + userList.contentHeight + PlasmaCore.Units.smallSpacing
-                                               + (newSessionButton.visible ? newSessionButton.height : 0)
-                                               + (lockScreenButton.visible ? lockScreenButton.height : 0)
-                                               + leaveButton.height
+            readonly property real implicitContentHeight: {
+                let h = currentUserItem.implicitHeight;
+                h += userList.contentHeight;
+                if (newSessionButton.visible) {
+                    h += newSessionButton.implicitHeight;
+                }
+                if (lockScreenButton.visible) {
+                    h += lockScreenButton.implicitHeight;
+                }
+                h += leaveButton.implicitHeight;
+            }
 
             anchors.fill: parent
-
             spacing: 0
 
             ListDelegate {