Use case

Currently, the DropdownMenu widget allows the user to:

1. Select an option from the dropdown list (triggering onSelected).
2. Type arbitrary text into the input field to perform searching or filtering.
3. Tap out of focus and leave the arbitrary text visible without selecting anything or triggering any callbacks.

This is confusing as the text field is the primary feedback for which item has been selected yet it no longer reflects that. See example.

Proposal

I believe the most comprehensive solution would be:

DropdownMenu(
    autoSelect: true,       // Automatically select highlighted menu item if focus lost
    incompleteTextBehavior: // If no highlighted menu item, or autoSelect: false
        DropdownIncompleteTextBehavior.ignore, // Leave text as-is,  don't emit (current behavior)
     or DropdownIncompleteTextBehavior.emit,   // Leave text as-is,  emit `onSelected(null)`
     or DropdownIncompleteTextBehavior.clear,  // Clear text,        emit `onSelected(null)`
     or DropdownIncompleteTextBehavior.reset,  // Reset to previous, emit `onSelected(previous)`
)

DropdownIncompleteTextBehavior.ignore could then be deprecated with a view to removing in future, forcing the user to do something more sensible.