Let the view list and explorer sidebar be resized vertically

The divider between the map's view list and the palette or noise
sidebar was one pixel tall, too thin to grab. It is now a 7 px band
with a grip that can be dragged up or down. Palettes and Noise share
the one split, so the divider stays put when switching between them.
Its starting height is measured once the sidebar is laid out.

Model used: Claude Opus 5.5

core/src/main/java/com/volmit/iris/core/gui/MapWindow.java

@@ -59,6 +59,7 @@ public final class MapWindow implements MapCanvas.Listener {
     private static final int CONTENT_MINIMUM_WIDTH = 320;
     private static final double PALETTE_RECENTER_DISTANCE_BLOCKS = 1D;
     private static final double VIEW_LIST_SHARE_WITH_EXPLORER = 0.34D;
+    private static final int EXPLORER_DIVIDER_HEIGHT = 7;
 
     private final JFrame frame;
     private final PaletteExplorerStart start;
@@ -70,6 +71,7 @@ public final class MapWindow implements MapCanvas.Listener {
     private final JPanel sidebar = new JPanel(new BorderLayout());
     private final JPanel inspectorSlot = new JPanel(new BorderLayout());
     private final JComponent viewListScroll;
+    private final JSplitPane explorerSplit = new JSplitPane(JSplitPane.VERTICAL_SPLIT);
     private final List<JComponent> mapControls = new ArrayList<>();
     private final List<JComponent> explorerControls = new ArrayList<>();
     private final List<JComponent> noiseControls = new ArrayList<>();
@@ -122,6 +124,7 @@ public final class MapWindow implements MapCanvas.Listener {
     }
 
     private void buildFrame() {
+        styleExplorerSplit();
         content.add(canvas, MAP_CARD);
         statusBars.add(canvas.statusBar(), MAP_CARD);
         sidebar.add(viewListScroll, BorderLayout.CENTER);
@@ -249,15 +252,27 @@ public final class MapWindow implements MapCanvas.Listener {
         }
     }
 
+    // Both explorers share one split, so the divider stays where it was dragged when switching between them.
     private JSplitPane viewListAbove(JComponent explorerSidebar) {
-        JSplitPane split = new JSplitPane(JSplitPane.VERTICAL_SPLIT, viewListScroll, explorerSidebar);
-        split.setBorder(BorderFactory.createEmptyBorder());
-        split.setResizeWeight(0D);
-        split.setDividerLocation(viewListHeightAboveExplorer());
-        return split;
+        boolean firstUse = explorerSplit.getBottomComponent() == null;
+        explorerSplit.setTopComponent(viewListScroll);
+        explorerSplit.setBottomComponent(explorerSidebar);
+        if (firstUse) {
+            EventQueue.invokeLater(() -> explorerSplit.setDividerLocation(viewListHeightAboveExplorer()));
+        }
+        return explorerSplit;
     }
 
-    // The explorer needs the room more than the view list, which scrolls once it is squeezed.
+    private void styleExplorerSplit() {
+        explorerSplit.setBorder(BorderFactory.createEmptyBorder());
+        explorerSplit.setResizeWeight(0D);
+        explorerSplit.setBackground(StudioTheme.PANEL_BACKGROUND);
+        explorerSplit.setDividerSize(StudioTheme.scale(EXPLORER_DIVIDER_HEIGHT));
+        explorerSplit.putClientProperty("FlatLaf.style", "style: grip");
+    }
+
+    // The explorer needs the room more than the view list, which scrolls once it is squeezed. Measured once the sidebar
+    // is laid out, since the list's rows are taller before they know their width.
     private int viewListHeightAboveExplorer() {
         int fullHeight = viewList.getPreferredSize().height + 4;
         if (sidebar.getHeight() < 1) {