base on tabbar minimum width to show scrollbuttons

be more deterministic and now usually kickoff never shows the
scroll buttons

BUG:335392

src/declarativeimports/plasmacomponents/qml/TabBar.qml

@@ -82,7 +82,6 @@ FocusScope {
     implicitWidth: layout.implicitWidth
     implicitHeight: layout.implicitHeight
 
-
     Item {
         id: buttonCutter
         anchors {
@@ -267,15 +266,13 @@ FocusScope {
         rows: 1
         columns: 1
         flow: layout.isHorizontal ? GridLayout.TopToBottom : GridLayout.LeftToRight
-        visible: layout.isHorizontal ? tabBarLayout.width > root.width : tabBarLayout.height > root.height
+        visible: layout.isHorizontal ? tabBarLayout.minimumWidth >= root.width : tabBarLayout.height >= root.minimumHeight
 
         anchors {
             right: layout.isHorizontal ? parent.right : undefined
             verticalCenter: layout.isHorizontal ? parent.verticalCenter : undefined
             bottom: !layout.isHorizontal ? parent.bottom : undefined
             horizontalCenter: !layout.isHorizontal ? parent.horizontalCenter : undefined
-            rightMargin: y
-            bottomMargin: y
         }
         ToolButton {
             height: Math.min(parent.height, parent.width)

src/declarativeimports/plasmacomponents/qml/private/TabBarLayout.qml

@@ -65,6 +65,9 @@ Item {
 
     property bool isHorizontal: (tabPosition != Qt.LeftEdge && tabPosition != Qt.RightEdge)
 
+    property int minimumWidth: 0
+    property int minimumHeight: 0
+
     Component.onCompleted: layoutTimer.restart()
     onChildrenChanged: layoutTimer.restart()
     onWidthChanged: layoutTimer.restart()
@@ -164,7 +167,7 @@ Item {
             if (tabGroup) {
                 tabGroup.currentTab = root.children[index].tab
             }
-            
+
             priv.tabBar.currentTab = root.children[index]
         }
 
@@ -231,6 +234,8 @@ Item {
             }
             root.implicitWidth = contentWidth
             root.implicitHeight = contentHeight
+            root.minimumWidth = itemWidth * visibleChildCount
+            root.minimumHeight = itemHeight * visibleChildCount
             if ( priv.tabBar.currentTab === null) {
                 //99% of the cases this loop will be length 1 but a tabbar can also have other children, such as Repeater
                 for (var i = 0; i < tabBarLayout.children.length; ++i) {