To prevent the default handling of the arrow keys by the TextField, you need to register the callback as a "TrickleDown" callback, meaning that you will receive the event before it is processed by the TextField. You can then have your own handling of it and stop the propagation of the event.

In your case, you want to keep the handling of the "Enter" as a "NoTrickleDown" callback (assuming you want to keep the double "Enter" to submit the value).

Here is an updated version of your callbacks that should achieve your desired result:

// *** THE POINT OF MY BUG SUBMISSION IS THIS CALLBACK ***
// Customize hotkeys for the input textfield
input.RegisterCallback<KeyDownEvent>(evt =>
{
// Hitting up and down advances through the input history to use prior submissions
if (evt.keyCode == KeyCode.UpArrow)
{
if (_inputHistory.Count > 0)
{
if (_inputHistoryIndex > 0)
{
_inputHistoryIndex--;
}
input.value = _inputHistory[_inputHistoryIndex];
input.textSelection.cursorIndex = input.textSelection.selectIndex = input.value?.Length ?? 0;
evt.StopImmediatePropagation();
}
}
else if (evt.keyCode == KeyCode.DownArrow)
{
if (_inputHistory.Count > 0)
{
if (_inputHistoryIndex < _inputHistory.Count - 1)
{
_inputHistoryIndex++;
}
input.value = _inputHistory[_inputHistoryIndex];
input.textSelection.cursorIndex = input.textSelection.selectIndex = input.value?.Length ?? 0;
evt.StopImmediatePropagation();
}
}
}, TrickleDown.TrickleDown);

input.RegisterCallback<KeyDownEvent>(evt =>
{
// Hitting enter submits the text of the input textfield
if (evt.keyCode == KeyCode.Return || evt.keyCode == KeyCode.KeypadEnter)
{
onSubmit();
}
});