13.0d 20210531.

Consider this screenshot:
Click image for larger version

Name:	Screen Shot 2021-06-01 at 09.38.33.png
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If you, in linear mode etiher

1. set autoWidth on the form or
2. set titleorientation to LEFT.

The textarea renders as above, if you set linearmode to true on the form.

code example:
Code:
public void onModuleLoad() {
Browser.setIsHandset(true);
DynamicForm form = new DynamicForm();
form.setBorder("5px solid red");

form.setLinearMode(true);
//form.setTitleOrientation(TitleOrientation.LEFT);//causes narrow textarea on mobile
form.setAutoWidth();//causes narrow textarea on mobile

TextItem text = new TextItem("text", "text");
FormItem area = new TextAreaItem("area1", "Area 1");
form.setItems(text, area);
form.draw();
}