Flutter textfield that auto expands when text is entered and then starts scrolling the text when a certain height is reached

Container(
    child: new ConstrainedBox(
        constraints: BoxConstraints(
            maxHeight: 300.0,
        ),
        child: TextField(
                    maxLines: null,
                ),
            ),
        ),
    ),
)

In older Flutter versions it was

Container(
    child: new ConstrainedBox(
        constraints: BoxConstraints(
            maxHeight: 300.0,
        ),
        child: new Scrollbar(
            child: new SingleChildScrollView(
                scrollDirection: Axis.vertical,
                reverse: true,
                child: new TextField(
                    maxLines: null,
                ),
            ),
        ),
    ),
)

Now we actually have minLines parameter of TextField, no workaround needed anymore.

TextField(
    minLines: 1,
    maxLines: 5,
)

The accepted answer by Gunter is good enough if you don't have any style for the TextField. But if you have at least an underline / bottom border for the TextField, it will disappear when scroll up.

My recommendation is to calculating the lines with TextPainter, then apply the calculated number of lines to TextField. Here's the code, replace your current TextField with LayoutBuilder :

LayoutBuilder(
    builder: (context, size){
      TextSpan text = new TextSpan(
        text: yourTextController.text,
        style: yourTextStyle,
      );

      TextPainter tp = new TextPainter(
          text: text,
          textDirection: TextDirection.ltr,
          textAlign: TextAlign.left,
      );
      tp.layout(maxWidth: size.maxWidth);

      int lines = (tp.size.height / tp.preferredLineHeight).ceil();
      int maxLines = 10;

      return TextField(
        controller: yourTextController,
        maxLines: lines < maxLines ? null : maxLines,
        style: yourTextStyle,
      );
    }
  )