It seems that a big bug is there (tested on Qt 4.8.0 and Qt 4.8.1)

here is code to test:

QTextEdit *editor = new QTextEdit();
QTextCursor cursor(editor->textCursor());
QTextBlockFormat blockFormat = cursor.blockFormat();
blockFormat.setLeftMargin(150);
cursor.setBlockFormat(blockFormat);
for (int i=0; i<100; i++) cursor.insertText("aaa aaaa ");

The cursor can't go on the right part of the text (150 pixels from the right side of the textedit), using arrow keys or mouse gives the same result.

i've built my project with Qt 4.7.4, the bug is not there
it's only in 4.8.0 and 4.8.1 it seems