Conditions to reproduce:

• no wrapMode specified on the TextArea
• the text doesn't have any spaces or newlines in it
• the text exceeds the width of the textarea

What's wrong :
-> no horizontal scrollbar shows up
-> mouse selection doesn't work on the right part of the text (the part that exceeds the width of the textarea).

If you specify "wrapMode: Text.NoWrap", it works well although the documentation says it's the default value.

Code snippet:

Rectangle {
    id: root
    width: 400
    height: 400

   TextArea {
            x:50
            y:50
            height: 100
            width: 300
            text: "TextWithoutSpacesTextWithoutSpacesTextWithoutSpacesTextWithoutSpacesTextWithoutSpacesTextWithoutSpacesTextWithoutSpacesTextWithoutSpaces"
        }
}