Basically, if you are using a multi-line text control (e.g. the status editor, or the non-Redactor message editor), you are unable to highlight any of the text without using the mouse exclusively.
The moment you release a key on the keyboard, the text is de-selected. So you cannot use Shift + Arrow Keys in order to select letters or words, or anything for that matter. You can use the mouse to highlight content, but if you use the keyboard - even so much as just tapping "Ctrl" without doing anything, you'll lose the selection again.
Have you ever heard of this happening? It does not seem to occur in the default style, but it does happen in our current style and is particularly annoying.