topcat: add CellViewWindow

The "Cell View" is a new drop-down menu option when a user right-clicks
on a cell in a TableViewerWindow. The Cell View is displayed at the
bottom after a divider, to make it distinct from column operations. When
clicked, it pops open a small window that displays the full contents of
that cell.

Author: fjebaker

topcat/src/main/uk/ac/starlink/topcat/CellViewWindow.java

@@ -0,0 +1,50 @@
+package uk.ac.starlink.topcat;
+
+import java.awt.Component;
+import java.awt.Dimension;
+import javax.swing.JTextArea;
+
+/**
+ * Window for defining up a mutually exclusive group of subsets
+ * based on the values of a given table expression.
+ *
+ * @author   Fergus Baker
+ * @since    06 Mar 2026
+ */
+public class CellViewWindow extends AuxWindow {
+    String cellString_;
+    JTextArea textArea_;
+
+    /**
+     * Constructor.
+     *
+     * Initialises the Cell View window without any display text.
+     *
+     * @param title The title for this window.
+     * @param parent The parent component.
+     */
+    @SuppressWarnings("this-escape")
+    public CellViewWindow( String title, Component parent ) {
+        super(title, parent);
+        textArea_ = new JTextArea( 5, 25 );
+        textArea_.setEditable( false );
+        textArea_.setLineWrap( true );
+        textArea_.setWrapStyleWord( true );
+
+        /* These cause the this-escape warning, but are perfectly safe in this
+         * context. */
+        setPreferredSize( new Dimension( 300, 200 ) );
+        getContentPane().add( textArea_ );
+        addHelp( null );
+    }
+
+    /**
+     * Used to set the text to display in the Cell View window.
+     *
+     * @param text Text to display in this component.
+     */
+    public void setText( String text ) {
+        textArea_.selectAll();
+        textArea_.replaceSelection( text );
+    }
+}

topcat/src/main/uk/ac/starlink/topcat/TableViewerWindow.java

@@ -563,6 +563,30 @@ public void actionPerformed( ActionEvent evt ) {
              popper.add( explodeAct );
         }
 
+        popper.addSeparator();
+
+        /* Get the current row that is being selected. */
+        final int jrow = rowSelectionModel_.getMinSelectionIndex();
+
+        /* Action to open the cell text in a viewer. */
+        Action viewCellAct =
+            new BasicAction( "View Cell", ResourceIcon.ZOOM_IN,
+                             "View the cell contents in a viewer." ) {
+                public void actionPerformed( ActionEvent evt ) {
+                    CellViewWindow cell_view =
+                        new CellViewWindow( "Cell Viewer", parent );
+                    TableModel tm = jtable_.getModel();
+                    String selectedCell = tm.getValueAt( jrow, jcol ).toString();
+                    cell_view.setText( selectedCell );
+                    cell_view.setVisible( true );
+                }
+            };
+
+        /* Only enable the Cell View option if something is selected. If no row
+         * column is selected, it will be displayed in deactived state. */
+        viewCellAct.setEnabled( jrow >= 0 && jcol >= 0 );
+        popper.add( viewCellAct );
+
         return popper;
     }