What is the relation between MediaQuery.of(context) and TextField cursor position in Flutter?

This is screen code:

import 'package:flutter/material.dart';

class Model {
  String? text;

  Model(this.text);
}

class TempDialog extends StatefulWidget {

  TempDialog({Key? key}) : super(key: key);

  @override
  State<TempDialog> createState() => _TempDialogState();
}

class _TempDialogState extends State<TempDialog> {

  final model = Model("This is test text");

  @override
  Widget build(BuildContext context) {
    var textController = TextEditingController(text: model.text);
    return AlertDialog(
      content: Container(
        width: MediaQuery
            .of(context)
            .size
            .width - 50,
        height: MediaQuery
            .of(context)
            .size
            .height - 300,
        child: Column(
          children: [
            TextField(
              textAlign: TextAlign.left,
              decoration: InputDecoration(
                contentPadding: EdgeInsets.all(10.0),
              ),
              keyboardType: TextInputType.multiline,
              maxLines: null,
              minLines: 4,
              controller: textController,
            )
          ],
        ),
      ),
    );
  }
}

class TempScreen extends StatefulWidget {

  TempScreen({Key? key}) : super(key: key);

  @override
  State<TempScreen> createState() => _TempScreenState();
}

class _TempScreenState extends State<TempScreen> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Temp screen"),
      ),
      body: Column(
        children: [
          TextButton(
              onPressed: (){
                showDialog(
                    context: context,
                    builder: (BuildContext context) {
                      return TempDialog();
                    }
                );
              },
              child: Text("Tap me"))
        ],
      ),
    );
  }
}

And this is the result:

As you see although a user taps somewhere in the middle of the text, after keyboard appears cursor moves to start (home). The problem is in these lines:

        width: MediaQuery
            .of(context)
            .size
            .width - 50,
        height: MediaQuery
            .of(context)
            .size
            .height - 300,

If we replace them with these :

        width: 200,
        height: 200,

everything works as expected.

Could anyone explain this relation between MediaQuery.of(context) and TextField cursor position?

I've created the same issue on web, calling setState and requesting new focus on TextField. The solution is to use textController outside the build method.

  final model = Model("This is test text");
  final FocusNode focusNode = FocusNode();
  late TextEditingController textController = TextEditingController.fromValue(
    TextEditingValue(text: model.text ?? ""),
  );