Bug 124202 [Field Assist] - decorated fields should not size the created control
The philosophy of DecoratedField is that the client creates the target control and accesses it with normal SWT coding. The decorated field manages the decorations but otherwise statys out of the way. This philosophy is violated when DecoratedField sets the width of the field to IDialogConstants.ENTRY_WIDTH. Sizing decisions should be made by the client of the decorated field, not the field itself.
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/DecoratedField.java
public Control getLayoutControl() {
		return form;
	}
	/**
	 * Create the parent composite and a form layout that will be used to manage
	 * decorations.
	 */
	private Composite createForm(Composite parent) {
		Composite composite = new Composite(parent, SWT.NO_FOCUS);
		composite.setLayout(new FormLayout());
		return composite;
	}
	/**
	 * Add any listeners needed on the target control.
	 */
	private void addControlListeners() {
		control.addDisposeListener(new DisposeListener() {
			public void widgetDisposed(DisposeEvent event) {
				if (hover != null)
					hover.dispose();
			}
		});
		control.addFocusListener(new FocusListener() {
			public void focusGained(FocusEvent event) {
				controlFocusGained();
			}
			public void focusLost(FocusEvent event) {
				controlFocusLost();
			}
		});