do not count Hidden nodes at the beginning of the scrollable (#6381)

shell/platform/android/io/flutter/view/AccessibilityBridge.java

@@ -720,17 +720,19 @@ class AccessibilityBridge
                     event.setMaxScrollX((int) max);
                 }
                 if (object.scrollChildren > 0) {
+                    // We don't need to add 1 to the scroll index because TalkBack does this automagically.
                     event.setItemCount(object.scrollChildren);
                     event.setFromIndex(object.scrollIndex);
-                    int visibleChildren = object.childrenInHitTestOrder.size() - 1;
-                    // We assume that only children at the end of the list can be hidden.
-                    assert(!object.childrenInHitTestOrder.get(object.scrollIndex).hasFlag(Flag.IS_HIDDEN));
-                    for (; visibleChildren >= 0; visibleChildren--) {
-                        SemanticsObject child = object.childrenInHitTestOrder.get(visibleChildren);
+                    int visibleChildren = 0;
+                    // handle hidden children at the beginning and end of the list.
+                    for (SemanticsObject child : object.childrenInHitTestOrder) {
                         if (!child.hasFlag(Flag.IS_HIDDEN)) {
                             break;
                         }
+                        visibleChildren += 1;
                     }
+                    assert(object.scrollIndex + visibleChildren <= object.scrollChildren);
+                    assert(!object.childrenInHitTestOrder.get(object.scrollIndex).hasFlag(Flag.IS_HIDDEN));
                     event.setToIndex(object.scrollIndex + visibleChildren);
                 }
                 sendAccessibilityEvent(event);