Do not create nodes in Tree that are not visible

[Chirag Mistry]

Tried to use the dragUntilVisible function in a reg test to find and click on an item, but the screen never scrolls. On investigation it seems that is because dragUntilVisible doesn’t drag the screen/scrollable until the widget of interest is visible, instead it drags the screen/scrollable until the widget of interest exists.

Using the same function on a ListView however does scroll until the widget of interest is visible, meaning that the widget while off screen does not exist. At least that is what I think is happening

This in turn means that there is potential inefficiency of the tree view in that it is creating widgets that are outside of the viewport.