SnapAttraction prefers wrong window

[ThomasAdam]

From @domivogt (via fvwm-workers@):

Snapping to the grid, other windows and screen edges hat a nasty
interaction that has been bugging me for years, but I don't know
how to fix it. Suppose you have this:

Style * SnapAttraction 12 Sametype, SnapGrid 4 4
EdgeResistance 500
Style * EdgeMoveDelay 500, EdgeMoveResistance 128
EdgeScroll 0 0

There's a button bar in the bottom left corner and another window
that is a little shorter than the screen height minus the buttons
height, say, one pixel shorter:

  +---------------------+
  |+--------+           | <--- small gap
  || window |           |
  ||        |           |
  ||        |           |
  ||        |           |
  ||        |           |
  ||        |           |
  |+--------+           |
  |+------------+       |
  || button bar |       |
  +---------------------+

When moving the window with the mouse it's difficult to snap the
window to the top edge of the screen because it prefers to snap to
the button bar instead. Try it out yourself. I just don't know
how to "fix" it.

[ThomasAdam]

The snapping code is really an unmaintainable mess. Some thoughts
on how it might be rewritten:

Situation:

1. There are three different snapping mechanisms:

a) Snapping to windows or icons.
b) Snapping to screen edges.
c) Snapping to the grid.

Each one has some distance defined at which snapping will occur.
(a) and (b) are set directly, the value for (c) is roughly half
the grid size.

2. The top border is in the vicinity of some locations it might
   snap to:

• The bottom border of the next window above it.
• The bottom border of the next window below it.
• Possibly it's directly on the bottom border.
• The top edge of the screen.
• The grid line above it.
• The grid line below it.

3. There needs to be some algorithm to decide to which of these
   coordinates the window should be snapped. The current algorithm
   treats all three mechanisms in separate code which can cause
   confusing interaction of windows jumping around while moving.

The three mechanisms are not equally important. For example, if
you have configured that windows would snap to screen edges at 16
pixels distance, and the grid size is 4, the snap grid should not
override the edge snapping at, say, 10 pixels distance.

4. The situation gets much more complicated because the bottom
   border of the window has completely different coordinates it can
   snap to.

5. The top or bottom border of the window may be off screen. The
   window should not snap to objects that are on another page.

--

Idea: At a given window position, gather a list of possible snap
coordinates and attraction distances for the top border (2).
Gather a similar list for the bottom border (4). The list for the
bottom border can actually be recalculated into top border
coordinates (if the bottom coordinate is y, the top coordinate is
(y - window height)).

Now run the algorithm (3) to decide which coordinate shall be used
for snapping. Treat window borders and screen edges as equally
important. Whatever value is closest is chosen. Snap to the grid
only if the window was not snapped to another window, icon or
screen edge.