Bug 201906 [Viewers] 3.4 ColumnViewerEditor leaks ViewerCell instances.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ColumnViewerEditor.java
if (nextCell != null) {
			if (isCellEditable(viewer, nextCell)) {
				rv = nextCell;
			} else {
				rv = searchNextCell(row, nextCell, originalCell, viewer);
			}
		} else {
			if ((feature & TABBING_CYCLE_IN_ROW) == TABBING_CYCLE_IN_ROW) {
				rv = searchNextCell(row, null, originalCell, viewer);
			} else if ((feature & TABBING_MOVE_TO_ROW_NEIGHBOR) == TABBING_MOVE_TO_ROW_NEIGHBOR) {
				ViewerRow rowBelow = row.getNeighbor(ViewerRow.BELOW, false);
				if (rowBelow != null) {
					rv = searchNextCell(rowBelow, null, originalCell, viewer);
				}
			}
		}
		return rv;
	}
	/**
	 * Position the editor inside the control
	 *
	 * @param w
	 *            the editor control
	 * @param item
	 *            the item (row) in which the editor is drawn in
	 * @param fColumnNumber
	 *            the column number in which the editor is shown
	 */
	protected abstract void setEditor(Control w, Item item, int fColumnNumber);
	/**