Bug 201906 [Viewers] 3.4 ColumnViewerEditor leaks ViewerCell instances.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ColumnViewerEditor.java
if (editorActivationListener != null
							&& !editorActivationListener.isEmpty()) {
						Object[] ls = editorActivationListener.getListeners();
						for (int i = 0; i < ls.length; i++) {
							((ColumnViewerEditorActivationListener) ls[i])
									.afterEditorDeactivated(tmp);
						}
					}
					this.cellEditor = null;
					this.cell = null;
				}
			} finally {
				inEditorDeactivation = false;
			}
		}
	}
	/**
	 * Enable the editor by mouse down
	 *
	 * @param event
	 */
	void handleEditorActivationEvent(ColumnViewerEditorActivationEvent event) {
		
		if (!event.cancel
				&& editorActivationStrategy.isEditorActivationEvent(event)) {
			if (cellEditor != null) {
				applyEditorValue();
			}
			this.cell = (ViewerCell) event.getSource();
			activateCellEditor(event);