collide is aware of fixed positions of the nodes

[rioj7]

If you have fixed position nodes (fx,fy !== null) that possibly overlap, the collision keeps these nodes fixed and we don't collide with a shadow version at d.x + d.vx, d.y + d.vy

[rioj7]

this will fix #213

I have recreated the tests from test/collide-test.js in a browser by implementing a very simple test runner.

The original code also fails the test forceCollide collides nodes and the 2 jiggle tests.

In the jiggle test forceCollide jiggles equal positions there are the lines

  assert.strictEqual(a.vx, -b.vx);
  assert.strictEqual(a.vy, -b.vy);

the test result says:

forceCollide jiggles equal positions
0.4397129164975401  0.4397129164975401 - values not strict equal
0.4082309018184576  0.4082309018184576 - values not strict equal

they are 16 decimal places equal, most likely the last bits (1 or 2) of the mantissa are different.
The assert should also compare with a delta.

The test forceCollide jiggles in a reproducible way assumes the random number generator produces the same number sequence for each test. Is this also true if you change the order of the tests?

If I try the modified code it has the same results for the test forceCollide collides nodes as the original code.
It should be because there are no nodes with fixed xy in the test and the modification only does something different for nodes with a fixed x-or-y.