Selection area incorrect when type Ctrl - Z

[banyudu]

Selection area keep correct when click Bold/Italic, but incorrect when type Ctrl - Z

Steps:

1. type "hello" in empty document, and select it.
2. click "Bold" button in toolbar, now it is hello, "hello" is selected.
3. type Ctrl - Z, now it is hello, only "llo" is selected.

When we type Ctrl - Z, selection area start/end does not change.