Problems rendering continious spaces in the end of line with text-decoration

Create a QTextEdit object textEdit and run

ui->textEdit->setText("<span style=\"text-decoration: underline; text-decoration-color: red; white-space: pre;\">    </span><br>"); 

Expected result: all the four spaces should be underlined, and these spaces can be selected

Actual result: only the first space is underlined, and these spaces cannot be selected

(I am not an English native speaker so I wish my grammar mistakes won't disturb understanding)