Below are two test cases that I expect to pass:

Resizing parent

a) expected and actual: distribute the delta across children
b) expected: resize itself to the new width - actual: parent width unchanged

    @Test
    public void testResizeParentColumn() {
        TableView table = new TableView();
        TableColumn first = new TableColumn("First Name");
        TableColumn last = new TableColumn("Last Name");
        TableColumn parent = new TableColumn("Parent");
        parent.getColumns().addAll(first, last);
        table.getColumns().addAll(parent);
        
        double initialWidth = 80;
        assertEquals(initialWidth, first.getWidth(), .1);
        assertEquals(initialWidth, last.getWidth(), .1);
        assertEquals(first.getWidth() + last.getWidth(), parent.getWidth(), .1);
        
        double delta = 100;
        assertTrue(table.resizeColumn(parent, delta));
        assertEquals(initialWidth + delta/2, first.getWidth(), .1);
        assertEquals("parent width must be same as sum of childrens width",
                first.getWidth() + last.getWidth(), parent.getWidth(), .1);
    }
 
Resizing child:

a) expected and actual: resizes the child without touching siblings
b) expected: resize parent to new combined width - actual: parent width unchanged
   
    @Test
    public void testResizeChildColumn() {
        TableView table = new TableView();
        TableColumn first = new TableColumn("First Name");
        TableColumn last = new TableColumn("Last Name");
        TableColumn parent = new TableColumn("Parent");
        parent.getColumns().addAll(first, last);
        table.getColumns().addAll(parent);
        
        double initialWidth = 80;
        assertEquals(initialWidth, first.getWidth(), .1);
        assertEquals(initialWidth, last.getWidth(), .1);
        assertEquals(first.getWidth() + last.getWidth(), parent.getWidth(), .1);
        
        double delta = 100;
        assertTrue(table.resizeColumn(first, delta));
        assertEquals(initialWidth + delta, first.getWidth(), .1);
        assertEquals(initialWidth, last.getWidth(), 0.1);
        assertEquals("parent width must be same as sum of childrens width",
                first.getWidth() + last.getWidth(), parent.getWidth(), .1);
    }