[Controls] Prevent the filter text field from being out of the window

The filter text field is now always placed at the position of the
attribute, meaning it is always in sight. The list of values is
then either dropped downwards or opened upwards depending on its
size and where there is the most free space.

meshroom/ui/qml/Controls/FilterComboBox.qml

@@ -66,15 +66,34 @@ ComboBox {
 
     popup: Popup {
         width: combo.width
-        implicitHeight: contentItem.implicitHeight 
+        implicitHeight: contentItem.implicitHeight
+
+        x: 0
+        y: 0
 
         onAboutToShow: {
             filterTextArea.forceActiveFocus()
 
-            if (mapToGlobal(popup.x, popup.y).y + root.implicitHeight * (model.length + 1) > _window.contentItem.height) {
-                y = -((combo.height * (combo.model.length + 1) > _window.contentItem.height) ? _window.contentItem.height*2/3 : combo.height * (combo.model.length + 1))
+            var dropDown = true
+            var posY = mapToGlobal(popup.x, popup.y).y
+
+            /* If the list will go out of the screen by dropping down AND if there is more space up than down,
+             * then open it upwards.
+             * Having both conditions allows to naturally drop down short lists that are visually located close
+             * to the lower border of the window, while opening upwards long lists that are in that same location
+             * AND opening these same lists downwards if they are located closer to the upper border.
+             */
+            if (posY + root.implicitHeight * (model.length * 1) > _window.contentItem.height
+                && posY > _window.contentItem.height / 2) {
+                dropDown = false
+            }
+
+            if (dropDown) {
+                listView.anchors.bottom = undefined
+                listView.anchors.top = filterTextArea.bottom
             } else {
-                y = 0
+                listView.anchors.bottom = filterTextArea.top
+                listView.anchors.top = undefined
             }
         }
 
@@ -141,9 +160,14 @@ ComboBox {
             clip: true
             anchors.left: parent.left
             anchors.right: parent.right
-            anchors.top: filterTextArea.bottom
 
-            implicitHeight: (combo.height * (combo.model.length + 1) > _window.contentItem.height) ? _window.contentItem.height*2/3 : contentHeight
+            implicitHeight: {
+                if (combo.height * (combo.model.length + 1) > _window.contentItem.height) {
+                    return _window.contentItem.height * 2 / 3
+                } else {
+                    return contentHeight
+                }
+            }
             model: combo.popup.visible ? combo.delegateModel : null
 
             ScrollBar.vertical: ScrollBar {