Bug 75988 [Preferences] IPreferencePage#performOk() returning false should prevent the dialog from closing
I think that if this method returns false, it should prevent the dialog from closing. Today, in order to perform validation I have to register key listeners to all fields an use #setValid(false) to disable the "OK" button.
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/PreferenceDialog.java
/**
	 * Layout for the page container.
	 *  
	 */
	private class PageLayout extends Layout {
		public Point computeSize(Composite composite, int wHint, int hHint, boolean force) {
			if (wHint != SWT.DEFAULT && hHint != SWT.DEFAULT)
				return new Point(wHint, hHint);
			int x = minimumPageSize.x;
			int y = minimumPageSize.y;
			Control[] children = composite.getChildren();
			for (int i = 0; i < children.length; i++) {
				Point size = children[i].computeSize(SWT.DEFAULT, SWT.DEFAULT, force);
				x = Math.max(x, size.x);
				y = Math.max(y, size.y);
			}
			if (wHint != SWT.DEFAULT)