(I set it against mutter, but I am unsure)
(Feel free to rephrase the summary)

Guided window expansion is excellent.
(I mean the maximized and half-screen maximized, by drag&drop)

In the case of half-screen maximize, one don't necessarily need exactly half of the screen. As an example:

- a note on the left side (gnote/Tomboy, or simply a text file),
- and a document on the right side (LibreOffice)
--> We rather want 1/3 and 2/3 split of the screen


Basically the generic use case would be:
1. a main application/document open
      an office document, a web page, etc.
2. a side application/document open
      a note, a folder content(nautilus), a todo app, etc.

  +-----+---------------+
  |     |               |
  | sup |   main app    |
  | port|               |
  | app |               |
  |     |               |
  +-----+---------------+


==> I propose that once both windows are tiled half/half,
    the middle border would be panable (is it the right word?):

  +----------+----------+
  |          |          |
  |          |          |
  |       <--|-->       |
  |          |          |
  |          |          |
  +----------+----------+

==>

  +------+--------------+
  |      |              |
  |      |              |
  |   <--|-->           |
  |      |              |
  |      |              |
  +------+--------------+