make the ribbon actually scroll down off the page etc

Program.cs

@@ -286,6 +286,9 @@ public class Photo {
 
   public Texture Texture() {
     if (texture == placeholder && image != null) {
+      // The texture needs to be created on the GL thread, so we instantiate
+      // it here (since this is called from OnRenderFrame), as long as the
+      // image is ready to go.
       texture = new Texture(image);
       image.Dispose();
       image = null;
@@ -365,7 +368,7 @@ public class UiGeometry {
   public UiGeometry(Vector2i windowSize) {
     WindowSize = windowSize;
 
-    int numThumbnails = 13;
+    int numThumbnails = 20;
     int thumbnailHeight = WindowSize.Y / numThumbnails;
     int thumbnailWidth = (int) 1.0 * thumbnailHeight * activeCamera.Resolution.X / activeCamera.Resolution.Y;
     for (int i = 0; i < numThumbnails; i++) {
@@ -467,6 +470,7 @@ public class Game : GameWindow {
   int VertexArrayObject;
   List<Photo> photos = new();
   int photoIndex = 0;
+  int ribbonIndex = 0;
   Shader shader = new();
   Matrix4 projection;
   float zoomLevel = 0f;
@@ -477,6 +481,7 @@ public class Game : GameWindow {
 
     KeyboardState input = KeyboardState;
 
+    // FIXME: add a confirm dialog before closing. (Also for the window-close button.)
     // Close when Escape is pressed.
     if (input.IsKeyDown(Keys.Escape)) {
       Close();
@@ -487,9 +492,7 @@ public class Game : GameWindow {
       for (int i = 0; i < geometry.ThumbnailBoxes.Count; i++) {
         Box2i box = geometry.ThumbnailBoxes[i];
         if (box.ContainsInclusive((Vector2i) MouseState.Position)) {
-          if (0 <= i && i < photos.Count) {
-            photoIndex = i;
-          }
+          photoIndex = Math.Clamp(ribbonIndex + i, 0, photos.Count - 1);
         }
       }
     }
@@ -594,7 +597,7 @@ public class Game : GameWindow {
       }
     }
 
-    for (int i = 0; i < 100 && i < photos.Count; i++) {
+    for (int i = 0; i < 40 && i < photos.Count; i++) {
       await Task.Run( () => { photos[i].Load(); });
     }
   }
@@ -627,11 +630,15 @@ public class Game : GameWindow {
     DrawTexture(active, photoBox); 
 
     // Draw thumbnail boxes.
+    ribbonIndex = Math.Clamp(photoIndex - (geometry.ThumbnailBoxes.Count - 1) / 2, 0, Math.Max(0, photos.Count - geometry.ThumbnailBoxes.Count));
     DrawFilledBox(geometry.ThumbnailBox, Color4.Black);
-    for (int i = 0; i < photos.Count && i < geometry.ThumbnailBoxes.Count(); i++) {
+    for (int i = 0; i < geometry.ThumbnailBoxes.Count; i++) {
+      if (ribbonIndex + i >= photos.Count) {
+        break;
+      }
       Box2i box = geometry.ThumbnailBoxes[i];
-      DrawTexture(photos[i].Texture(), box);
-      if (i == photoIndex) {
+      DrawTexture(photos[ribbonIndex + i].Texture(), box);
+      if (ribbonIndex + i == photoIndex) {
         DrawBox(box, 5, Color4.Black);
         DrawBox(box, 3, Color4.White);
       }