Please describe the problem:
GNOME Desktop applications have two cut buffers, one is the implicit selection
buffer used by selecting text with the left mouse button and pasting with the
middle mouse button, the other is the explicit cut buffer used by selecting text
and pressing Control+C or by using the right mouse button context menu and
selecting Copy from the menu.

Selecting text in the To or CC text fields in the compose dialog should place
that text in the implicit selection buffer, but it also erases the contents of
the explicit cut buffer.

Steps to reproduce:
1. Select a message in the message list
2. Right click on an address field in the gray message header block and select
Copy from the menu
3. Click on the Reply button in the toolbar
4. Triple-click in the To field then press Delete on the keyboard to make the
field empty
5a. Right click inside the empty To field and select Paste
OR
5b. Press Control+V




Actual results:
Nothing.

Expected results:
The previously copied address is inserted into the To field.

Does this happen every time?
Yes.

Other information:
Probably related to bug 300429.  I view this as an important usability bug.