RE: purpose and usage of secondary-selection ???

[Drew Adams <drew.adams@oracle.com>]

> Sometimes while typing on my laptop I accidentally brush against the
> touchpad and inadvertantly mess up something.  Today, while not even
> emacs, this happened, affecting what happened in an emacs frame which
> previously didn't even have the focus.
> 
> What I ended up in that emacs frame was that a random section of text
> was set to background yellow.  Investigating further, I found that text
> with the yellow background was a secondary-selection.  But I couldn't
> find anything in the Help screens describing anything about it, except
> its properties.  So here are my questions:
> 
> What is/are the purpose(s) of a secondary-selection?
> 
> What can be done with a secondary-selection...? and how to do them?
> 
> How do I undo this secondary-selection... i.e., make the selection go
> away without changing the file/buffer in any way?

1. Emacs Wiki is your friend:
https://www.emacswiki.org/emacs/SecondarySelection

2. Its characteristics tell you its advantages and disadvantages.

a. It is separate from the region.  So you have two pieces of text you
   can paste/yank.

b. It is unrelated to point (the cursor position).  The region always has
   point as one of its end positions (mark is the other end).

3. A good use case: Selective replacement of things (same thing or
   different things) with the same text.  Select some text, then yank
   the secondary selection.  Repeat - anywhere; any text to replace.

   You cannot do this with the region text, because as soon as you
   select the text to replace you've redefined the region, and so
   also the text to yank.

4. For the secondary selection to really be useful, IMHO, you need to
   bind yanking it to a keyboard key, not just to a mouse action.

5. Library `second-sel.el' can help make the secondary selection
   more useful.
   https://www.emacswiki.org/emacs/SecondarySelection#secondary-sel.el

6. See the Emacs manual, node `Secondary Selection' for more info.
   http://www.gnu.org/software/emacs/manual/html_node/emacs/Secondary-Selection.html

7. To make the secondary selection empty ("get rid of it"): Click
   `M-mouse-1' (don't drag).