Fixes #4731: Take the position where the user clicked the handle into account when calculating the new scrollbar position.

Also fixes: The scrollbar did not subtract the handle height when determining the available space for moving the handle, thus introducing drift the further down a user scrolled.
Also fixes: The scrollbar did not resize it's trackbar when the ME inventory screens were resized and the number of rows changed, same for craft confirm.

src/main/java/appeng/client/gui/widgets/Scrollbar.java

@@ -25,9 +25,20 @@ import com.mojang.blaze3d.matrix.MatrixStack;
 import net.minecraft.client.gui.AbstractGui;
 import net.minecraft.client.renderer.Rectangle2d;
 import net.minecraft.util.ResourceLocation;
+import net.minecraft.util.math.MathHelper;
 
 import appeng.client.gui.AEBaseScreen;
 
+/**
+ * Implements a vertical scrollbar using Vanilla's scrollbar handle texture from
+ * the creative tab.
+ * <p>
+ * It is expected that the background of the UI contains a pre-baked scrollbar
+ * track border, and that the exact rectangle of that track is set on this
+ * object via {@link #setLeft(int)}, {@link #setTop(int)} and
+ * {@link #setHeight(int)}. While the width of the track can also be set, the
+ * drawn handle will use vanilla's sprite width (see {@link #HANDLE_WIDTH}.
+ */
 public class Scrollbar extends AbstractGui implements IScrollSource {
 
     /**
@@ -87,6 +98,12 @@ public class Scrollbar extends AbstractGui implements IScrollSource {
      * True if the scrollbar's handle is currently being dragged.
      */
     private boolean dragging;
+    /**
+     * The y-coordinate relative to the upper edge of the scrollbar handle, where
+     * the user pressed the mouse button to drag. While dragging, this is applied as
+     * an offset to the effective scrollbar position.
+     */
+    private int dragYOffset;
 
     private final EventRepeater eventRepeater = new EventRepeater(Duration.ofMillis(250), Duration.ofMillis(150));
 
@@ -124,6 +141,9 @@ public class Scrollbar extends AbstractGui implements IScrollSource {
      * of the scrollbar's track.
      */
     private int getHandleYOffset() {
+        if (getRange() == 0) {
+            return 0;
+        }
         int availableHeight = this.height - HANDLE_HEIGHT;
         return (this.currentScroll - this.minScroll) * availableHeight / this.getRange();
     }
@@ -203,6 +223,11 @@ public class Scrollbar extends AbstractGui implements IScrollSource {
             return false;
         }
 
+        // Do nothing when there's no range, but swallow the event
+        if (getRange() == 0) {
+            return true;
+        }
+
         int handleYOffset = getHandleYOffset();
 
         if (relY < handleYOffset) {
@@ -213,6 +238,7 @@ public class Scrollbar extends AbstractGui implements IScrollSource {
         } else if (relY < handleYOffset + HANDLE_HEIGHT) {
             // Clicks on the handle will initiate dragging it
             this.dragging = true;
+            this.dragYOffset = relY - handleYOffset;
         } else {
             // Clicks below the handle will page down, repeatedly
             pageDown();
@@ -233,15 +259,24 @@ public class Scrollbar extends AbstractGui implements IScrollSource {
             return;
         }
 
-        if (y > this.displayY && y <= this.displayY + this.height) {
-            this.currentScroll = (int) (y - this.displayY);
-            this.currentScroll = this.minScroll + ((this.currentScroll * 2 * this.getRange() / this.height));
-            this.currentScroll = (this.currentScroll + 1) >> 1;
-            this.applyRange();
-        }
+        // Compute the position of the mouse (adjusted for where it grabbed the handle,
+        // so as if it grabbed
+        // the upper edge of it) within the scrollable area of the track (minus the
+        // handle height).
+        double handleUpperEdgeY = y - this.displayY - this.dragYOffset;
+        double availableHeight = this.height - HANDLE_HEIGHT;
+        double position = MathHelper.clamp(handleUpperEdgeY / availableHeight, 0.0, 1.0);
+
+        this.currentScroll = this.minScroll + (int) Math.round(position * this.getRange());
+        this.applyRange();
     }
 
     public void wheel(double delta) {
+        // Do nothing when there's no range
+        if (getRange() == 0) {
+            return;
+        }
+
         delta = Math.max(Math.min(-delta, 1), -1);
         this.currentScroll += delta * this.pageSize;
         this.applyRange();