I agree with Linq here, that this is user un-friendly. I would have a "Copy" command button next to the textbox to be copied. Although this would require an additional line of code "FirstTextBox.SetFocus" between lines 2 & 3, it would not do the automatic copy when such would be a hinderance.
The Click Event occurs when a mouse button is pressed and released. Therefore, a user could select part of the text in FirstTextBox, but the entire text would be copied to the Clipboard. This is a behavior not standard to Windows, and because it is unexpected, might result in user-error.
Can't really thik of a valid reason for doing this, but this code will work:
I can and do use variations of this logic to lessen the number of keystrokes/mouse operations where copy/paste is used very frequently between related textboxes. However, I am VERY careful to avoid conflicts between this behavior and the Windows Clipboard.