Terms can be lost off-screen on very large expressions

[EthanWJohnson]

Seen on Win 10 Chrome

This seems to be a combination of two behaviors; first, when adding multiple-term expressions together the dropped expression will always add itself to the right of the anchor expression, and second, the anchor expression can be moved while the dropped expression is in transit, cancelling their merge.

Steps to do;
1.) Make very large expression. Drag it as far to the right as possible.
2.) Make another expression of 2 or more terms.
3.) Drop second expression on large one. It will move to place itself on the rightmost of the large expression.
4.) Move the large expression away immediately after dropping the second one.

The second expression is now lost off-screen. You can't get it back either without resizing the window to be very long and thin or resetting; putting the large expression to it pulls it to the right of the lost expression, losing that one too.

Related to phetsims/tasks#860

Debug Info:
Name: ‪Expression Exchange‬
URL: http://www.colorado.edu/physics/phet/dev/html/expression-exchange/1.1.0-dev.3/expression-exchange_en.html
Version: 1.1.0-dev.3 2017-06-13 18:51:38 UTC
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[jbphet]

I've added code that recognizes this situation and retrieves the lost expression. I'll leave this open and unassigned for now, and will have it tested on the next release.

[phet-steele]

@jbphet, perform the same procedure as above but replace step 4 with something else:

1.) Make very large expression. Drag it as far to the right as possible.
2.) Make another expression of 2 or more terms.
3.) Drop second expression on large one. It will move to place itself on the rightmost of the large expression.
4.) Cut the larger expression.

The other expression will be lost off screen. Performed in 1.1.0-dev.4

[jbphet]

Added code to prevent combining if expression is gone when destination reached. Ready for test.

[EthanWJohnson]

Disregarding being difficult to attempt, i'm unable to replicate the bug using either method, so it seems like its fixed and should stay fixed unless an equal but opposite code change makes it happen again.