Prevent DialogFragment from Resizing/Collapsing when Keyboard is Shown

The key is probably in the creation of the Dialog. When you extend DialogFragment, you just have to create a new Object of that type, call show on it and that's it. In onCreateView of that fragment you inflate your ordinary fragment layout file.

So an app that creates dialogs with custom fragments that don't resize when the soft input is opened, even on API level 30 (and up, probably), could look like this:

MainActivity:

public class MainActivity extends androidx.appcompat.app.AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        findViewById(R.id.show_dialog).setOnClickListener(v -> {
            new LoginFragment().show(getSupportFragmentManager(), "login");
        });
    }
}

LoginFragment:

public class LoginFragment extends androidx.fragment.app.DialogFragment.DialogFragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_dialogcontent, container, false);
    }
}

activity_main:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/show_dialog"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Show dialog"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

fragment_dialogcontent:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:minWidth="300dp"
    android:padding="16dp">

    <TextView
        android:id="@+id/login_title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Login"
        android:textAppearance="@style/TextAppearance.AppCompat.Large"
        app:layout_constraintBottom_toTopOf="@+id/login_description"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />

    <TextView
        android:id="@+id/login_description"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:text="Please login"
        app:layout_constraintBottom_toTopOf="@+id/login_username"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/login_title" />

    <com.google.android.material.textfield.TextInputLayout
        android:id="@+id/login_username"
        style="@style/Widget.MaterialComponents.TextInputLayout.FilledBox.Dense"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="24dp"
        android:hint="Username"
        app:errorEnabled="true"
        app:layout_constraintBottom_toTopOf="@+id/login_password"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/login_description">

        <com.google.android.material.textfield.TextInputEditText
            android:id="@+id/login_edittext_username"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:autofillHints="username"
            android:inputType="text|number" />
    </com.google.android.material.textfield.TextInputLayout>

    <com.google.android.material.textfield.TextInputLayout
        android:id="@+id/login_password"
        style="@style/Widget.MaterialComponents.TextInputLayout.FilledBox.Dense"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:hint="Password"
        app:endIconMode="password_toggle"
        app:errorEnabled="true"
        app:layout_constraintBottom_toTopOf="@+id/login_button_ok"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/login_username">

        <com.google.android.material.textfield.TextInputEditText
            android:id="@+id/login_edittext_password"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:autofillHints="password"
            android:inputType="textPassword" />
    </com.google.android.material.textfield.TextInputLayout>

    <Button
        android:id="@+id/login_button_ok"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="Login"
        app:layout_constraintBottom_toTopOf="@+id/login_button_cancel"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/login_password" />

    <Button
        android:id="@+id/login_button_cancel"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="Cancel"
        app:layout_constraintBottom_toTopOf="@+id/login_register_link"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/login_button_ok" />

    <com.google.android.material.button.MaterialButton
        android:id="@+id/login_register_link"
        style="@style/Widget.MaterialComponents.Button.TextButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="24dp"
        android:text="Register"
        android:textAppearance="@style/TextAppearance.AppCompat.Small"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/login_button_cancel" />

</androidx.constraintlayout.widget.ConstraintLayout>