Unsortable item can be sorted in some cases

[theluanr]

Describe the bug
Unsortable item can be sorted in some cases.

To Reproduce
Steps to reproduce the behavior:
Way one:

1. Drag a unsortable item to another unsortable item (no visual effect)
2. Both of them can be sorted now

Way Two:
1.A sortable item with index [n], and a unsortable item with index [n+1]
2.Drag the sortable cross the unsortable item (change their index)
3.Repeat step2 (reset their index)
4. The unsortable item can be sorted now

Expected behavior
Make sure unsortable item can't be sorted.

Test Case
Please copy this codepen and create a working test case showing the experienced bug: https://codepen.io/pen/?template=GRoQRxo
Has tested, it exists.

System (please complete the following information):

• OS: win10
• Browser chrome 100.0.4896.60
• Version v0.13.1

Additional context
None

[theluanr]

Way Three:
1.A unsortable item with index [n], and a sortable item with index [m] (n>m)
2.Drag a unsortable item to a sortable item (no visual effect)
3.The unsortable item can be sorted now

[theluanr]

It works on "items: ':not([unsortable item selector])'", not for "sortable('[[selector]]', 'disable')".
I suggest it need a new attribute which equivalent to [draggable="false"] when init, or block event when dragging a unsortable item.
Using "$('[selector]').attr('draggable', false);" to hack it temporary ...

[lukasoppermann]

If you can send a PR to fix this, that would be awesome.