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Targets

Accidentally activated DND shouldn't affect selection.
It should be possible to DND multiple (possibly discontinuous) items.
Possibilities of making a selection shouldn't be hindered by DND being activated; and don't interfere.

A mouse pressed/dragged sequence on a selected item may be start of a DND operation; don't change the selection then.
Modifiers:
Ctrl typically would both a) toggle (i.e. possibly unselect) the item's selection state and b) mean different operation in the DND operation to come.
Wait only for a specific time for a drag?
Shift would "extend" selection; should definitely not start DND then.
"dragging" for selection could only happen by starting on unselected items (and then DND shouldn't start!).

(doesn't really belong here)

Target cannot access the Transferable during dragging. All necessary information must be clumsily encoded in the data flavor. (#4248542)
Target cannot access source during dragging. At least it has to know whether the target is the same component (or model). This has to be encoded in the data flavor, or must be stored somewhere globally. (#4272503)
1.4: selection is misused for indication insertion cursor (#4468566), but not powerful enough (besides a) complete confusion if dragging within the same component, b) being not properly restored (ListSelectionModel.lead/anchor).