Add helper function PenpotUtils.addFlexLayout for adding a layout without changing relative child positions

Resolves #33

Additional changes: Improve prompts pertaining to layouts

mcp-server/data/prompts.yml

@@ -88,11 +88,10 @@ initial_instructions: |
   Boards can have layout systems that automatically control the positioning and spacing of their children:
 
     * **Flex Layout**: A flexbox-style layout system
-      - Add to a board with `board.addFlexLayout(): FlexLayout`; instance then accessibly via `board.flex`;
-        Check with: `if (board.flex) { ... }`
       - Properties: `dir`, `rowGap`, `columnGap`, `alignItems`, `justifyContent`;
          - `dir`: "row" | "column" | "row-reverse" | "column-reverse"
-      - Padding: `topPadding`, `rightPadding`, `bottomPadding`, `leftPadding`, or combined `verticalPadding`, `horizontalPadding`
+         - Padding: `topPadding`, `rightPadding`, `bottomPadding`, `leftPadding`, or combined `verticalPadding`, `horizontalPadding`
+         - To modify spacing: adjust `rowGap` and `columnGap` properties, not individual child positions
       - When a board has flex layout, 
          - child positions are controlled by the layout system, not by individual x/y coordinates;
            appending or inserting children automatically positions them according to the layout rules.
@@ -104,10 +103,14 @@ initial_instructions: |
         of the `children` array for dir="column" or dir="row", which is what you want. So call it in the order of visual appearance.
         To insert at a specific index, use `board.insertChild(index, shape)`, bearing in mind the reversed order for dir="column" 
         or dir="row".
-      - To modify spacing: adjust `rowGap` and `columnGap` properties, not individual child positions
+      - Add to a board with `board.addFlexLayout(): FlexLayout`; instance then accessible via `board.flex`.
+        IMPORTANT: When adding a flex layout to a container that already has children, 
+        use `penpotUtils.addFlexLayout(container, dir)` instead! This preserves the existing visual order of children.
+        Otherwise, children will be arbitrarily reordered when the children order suddenly determines the display order. 
+      - Check with: `if (board.flex) { ... }`
     
     * **Grid Layout**: A CSS grid-style layout system
-      - Add to a board with `board.addFlexLayout(): FlexLayout`; instance then accessibly via `board.grid`;
+      - Add to a board with `board.addGridLayout(): GridLayout`; instance then accessibly via `board.grid`;
         Check with: `if (board.grid) { ... }`
       - Properties: `rows`, `columns`, `rowGap`, `columnGap`
       - Children are positioned via 1-based row/column indices

penpot-plugin/src/PenpotUtils.ts

@@ -1,4 +1,4 @@
-import { Fill, FlexLayout, GridLayout, Page, Rectangle, Shape } from "@penpot/plugin-types";
+import { Board, Fill, FlexLayout, GridLayout, Page, Rectangle, Shape } from "@penpot/plugin-types";
 
 export class PenpotUtils {
     /**
@@ -198,6 +198,37 @@ export class PenpotUtils {
         shape.y = shape.parent.y + parentY;
     }
 
+    /**
+     * Adds a flex layout to a container while preserving the visual order of existing children.
+     * Without this, adding a flex layout can arbitrarily reorder children.
+     *
+     * The method sorts children by their current position (x for "row", y for "column") before
+     * adding the layout, then reorders them to maintain that visual sequence.
+     *
+     * @param container - The container (board) to add the flex layout to
+     * @param dir - The layout direction: "row" for horizontal, "column" for vertical
+     * @returns The created FlexLayout instance
+     */
+    public static addFlexLayout(container: Board, dir: "column" | "row"): FlexLayout {
+        // obtain children sorted by position (ascending)
+        const children = "children" in container && container.children ? [...container.children] : [];
+        const sortedChildren = children.sort((a, b) => (dir === "row" ? a.x - b.x : a.y - b.y));
+
+        // add the flex layout
+        const flexLayout = container.addFlexLayout();
+        flexLayout.dir = dir;
+
+        // reorder children to preserve visual order; since the children array is reversed
+        // relative to visual order for dir="column" or dir="row", we insert each child at
+        // index 0 in sorted order, which places the first (smallest position) at the highest
+        // index, making it appear first visually
+        for (const child of sortedChildren) {
+            child.setParentIndex(0);
+        }
+
+        return flexLayout;
+    }
+
     /**
      * Analyzes all descendants of a shape by applying an evaluator function to each.
      * Only descendants for which the evaluator returns a non-null/non-undefined value are included in the result.