FLEX-26478
CAUSE:
When replacing ContainerControllers (triggered when the text changes in a TextField / TextArea), StandardFlowComposer tried to dispose of the discarded ones first. It did this by calling clearSelectionShapes() and setRootElement(null) on the ContainerControllers. However, if that controller was already listening for mouse scroll events (when the user selects text and moves the mouse outside the text field in such a way that causes the text to scroll), this listener wasn't removed. Which meant that the next time scrollTimerHandler() was called (on the inactive ContainerController), textFlow was null due to the call to setRootElement(null).

SOLUTION:
StandardFlowComposer now calls a new method 'discard' on the ContainerControllers which are not needed anymore. This method now also stops the selection scroll timer ('_scrollTimer').