feat: fractional indexing (#7359)

* Introducing fractional indices as part of `element.index`

* Ensuring invalid fractional indices are always synchronized with the array order

* Simplifying reconciliation based on the fractional indices

* Moving reconciliation inside the `@excalidraw/excalidraw` package

---------

Co-authored-by: Marcel Mraz <marcel@excalidraw.com>
Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

packages/excalidraw/actions/actionDuplicateSelection.tsx

@@ -31,6 +31,7 @@ import {
   excludeElementsInFramesFromSelection,
   getSelectedElements,
 } from "../scene/selection";
+import { syncMovedIndices } from "../fractionalIndex";
 
 export const actionDuplicateSelection = register({
   name: "duplicateSelection",
@@ -90,6 +91,7 @@ const duplicateElements = (
   const newElements: ExcalidrawElement[] = [];
   const oldElements: ExcalidrawElement[] = [];
   const oldIdToDuplicatedId = new Map();
+  const duplicatedElementsMap = new Map<string, ExcalidrawElement>();
 
   const duplicateAndOffsetElement = (element: ExcalidrawElement) => {
     const newElement = duplicateElement(
@@ -101,6 +103,7 @@ const duplicateElements = (
         y: element.y + GRID_SIZE / 2,
       },
     );
+    duplicatedElementsMap.set(newElement.id, newElement);
     oldIdToDuplicatedId.set(element.id, newElement.id);
     oldElements.push(element);
     newElements.push(newElement);
@@ -238,9 +241,10 @@ const duplicateElements = (
   }
 
   // step (3)
-
   const finalElements = finalElementsReversed.reverse();
 
+  syncMovedIndices(finalElements, arrayToMap([...oldElements, ...newElements]));
+
   // ---------------------------------------------------------------------------
 
   bindTextToShapeAfterDuplication(