bug#41156: Updated description

[Neil Okamoto]

After further testing, I wanted to clarify that this occurs even without 
linum-mode.  It occurs whenever there are window margins to either side of the 
vertical divider.

So, e.g. 

$ emacs -nw -q
    M-x xterm-mouse-mode
    C-x b *scratch*
    C-x 3

    Use the mouse to drag the vertical divider right and left? Succeeds.

Now set a right margin on the left window:

    (set-window-margins (selected-window) 0 2)

    Use the mouse to drag the vertical divider to the left?
    Fails with the message “<right-margin> <mouse-movement> is undefined”

    (set-window-margins (selected-window) 0 0)

    And mouse-movement is permitted again.

Similarly, setting a left margin on the right window:

    (set-window-margins (selected-window) 2 0)

    Use the mouse to drag the vertical divider to the right?
    Fails with the message “<left-margin> <mouse-movement> is undefined”

    (set-window-margins (selected-window) 0 0)

    And mouse-movement is permitted again.