In ObjectPal, one of my favorite tricks was to capture the mouse down event,
fill a hidden textbox with the contents of a subtable record the user is
clicking on, then move the hidden textbox over top the subtable and make the
textbox visible. Next, the textbox would track the movements of the mouse
as the user held down the mouse button and hovered over a second subtable.
Upon releasing the mouse button, a new record would be created to relate the
two pieces of information. For example I used this technique for assigning
resources to a project, or students to a classroom.

If a user pressed down the mouse button over a subtable, how can I know
which record they're selecting?
If they release over a second subtable, how can I know which record they're
dropping on?
How can I capture fine increments of mouse movement to position my textbox?