allow mouse forward/back buttons to advance photoIndex. simplify photoIndex logic a bit

Program.cs

@@ -487,25 +487,57 @@ public class Game : GameWindow {
       Close();
     }
 
+    // Track keyboard repeat times for advancing up/down.
+    if (!input.IsKeyDown(Keys.Down)) {
+      downTimer = Int64.MaxValue;
+    }
+
+    if (!input.IsKeyDown(Keys.Up)) {
+      upTimer = Int64.MaxValue;
+    }
+
     // Look for mouse clicks on thumbnails.
-    if (MouseState.IsButtonPressed(0)) {
+    //
+    // Note that we don't bounds-check photoIndex until after all the possible
+    // inputs that might affect it. That simplifies this logic significantly.
+    if (MouseState.IsButtonPressed(MouseButton.Button1)) {
       for (int i = 0; i < geometry.ThumbnailBoxes.Count; i++) {
         Box2i box = geometry.ThumbnailBoxes[i];
         if (box.ContainsInclusive((Vector2i) MouseState.Position)) {
-          photoIndex = Math.Clamp(ribbonIndex + i, 0, photos.Count - 1);
+          photoIndex = ribbonIndex + i;
         }
       }
     }
 
-    // Track keyboard repeat times for advancing up/down.
-    if (!input.IsKeyDown(Keys.Down)) {
-      downTimer = Int64.MaxValue;
+    if (MouseState.IsButtonPressed(MouseButton.Button4)) {
+      photoIndex--;
     }
 
-    if (!input.IsKeyDown(Keys.Up)) {
-      upTimer = Int64.MaxValue;
+    if (MouseState.IsButtonPressed(MouseButton.Button5)) {
+      photoIndex++;
+    }
+
+    if (MouseState.ScrollDelta.Y < 0) {
+      photoIndex++;
     }
 
+    if (MouseState.ScrollDelta.Y > 0) {
+      photoIndex--;
+    }
+
+    // FIXME: make a proper Model class for tracking the state of the controls?
+    if (input.IsKeyPressed(Keys.Down) || now > downTimer) {
+      downTimer = now + 10000 * 200;
+      photoIndex++;
+    }
+    if (input.IsKeyPressed(Keys.Up) || now > upTimer) {
+      upTimer = now + 10000 * 200;
+      photoIndex--;
+    }
+
+    // Make sure the photoIndex is actually valid.
+    photoIndex = Math.Clamp(photoIndex, 0, photos.Count - 1);
+
     if (input.IsKeyDown(Keys.D0) || input.IsKeyDown(Keys.GraveAccent)) {
       zoomLevel = 0f;
     }
@@ -529,20 +561,6 @@ public class Game : GameWindow {
     if (input.IsKeyDown(Keys.D5)) {
       zoomLevel = 16f;
     }
-
-    // FIXME: make a proper Model class for tracking the state of the controls?
-    if (input.IsKeyPressed(Keys.Down) || now > downTimer) {
-      if (photoIndex < photos.Count - 1) {
-        downTimer = now + 10000 * 200;
-        photoIndex++;
-      }
-    }
-    if (input.IsKeyPressed(Keys.Up) || now > upTimer) {
-      if (photoIndex > 0) {
-        upTimer = now + 10000 * 200;
-        photoIndex--;
-      }
-    }
   }
 
   protected override async void OnLoad() {