shift for additive selection

packages/excalidraw/components/App.tsx

@@ -6562,6 +6562,7 @@ class App extends React.Component<AppProps, AppState> {
       this.lassoTrail.startPath(
         pointerDownState.origin.x,
         pointerDownState.origin.y,
+        event.shiftKey,
       );
     } else if (this.state.activeTool.type === "text") {
       this.handleTextOnPointerDown(event, pointerDownState);
@@ -7020,7 +7021,10 @@ class App extends React.Component<AppProps, AppState> {
   }
 
   private clearSelectionIfNotUsingSelection = (): void => {
-    if (this.state.activeTool.type !== "selection") {
+    if (
+      this.state.activeTool.type !== "selection" &&
+      this.state.activeTool.type !== "lasso"
+    ) {
       this.setState({
         selectedElementIds: makeNextSelectedElementIds({}, this.state),
         selectedGroupIds: {},
@@ -8261,7 +8265,8 @@ class App extends React.Component<AppProps, AppState> {
           selectedElements.length > 0 &&
           !pointerDownState.withCmdOrCtrl &&
           !this.state.editingTextElement &&
-          this.state.activeEmbeddable?.state !== "active"
+          this.state.activeEmbeddable?.state !== "active" &&
+          this.state.activeTool.type !== "lasso"
         ) {
           const dragOffset = {
             x: pointerCoords.x - pointerDownState.origin.x,
@@ -8611,7 +8616,11 @@ class App extends React.Component<AppProps, AppState> {
           this.lassoTrail.endPath();
           selectionSwitch = false;
         } else {
-          this.lassoTrail.addPointToPath(pointerCoords.x, pointerCoords.y);
+          this.lassoTrail.addPointToPath(
+            pointerCoords.x,
+            pointerCoords.y,
+            event.shiftKey,
+          );
         }
       } else {
         // It is very important to read this.state within each move event,

packages/excalidraw/lasso/index.ts

@@ -26,6 +26,7 @@ export class LassoTrail extends AnimatedTrail {
   private worker: Worker | null = null;
   private elementsSegments: Map<string, LineSegment<GlobalPoint>[]> | null =
     null;
+  private keepPreviousSelection: boolean = false;
 
   constructor(animationFrameHandler: AnimationFrameHandler, app: App) {
     super(animationFrameHandler, app, {
@@ -49,7 +50,7 @@ export class LassoTrail extends AnimatedTrail {
     });
   }
 
-  startPath(x: number, y: number) {
+  startPath(x: number, y: number, keepPreviousSelection = false) {
     // clear any existing trails just in case
     this.endPath();
 
@@ -57,6 +58,16 @@ export class LassoTrail extends AnimatedTrail {
     this.intersectedElements.clear();
     this.enclosedElements.clear();
 
+    this.keepPreviousSelection = keepPreviousSelection;
+
+    if (!this.keepPreviousSelection) {
+      this.app.setState({
+        selectedElementIds: {},
+        selectedGroupIds: {},
+        selectedLinearElement: null,
+      });
+    }
+
     try {
       this.worker = new LassoWorker();
 
@@ -80,6 +91,12 @@ export class LassoTrail extends AnimatedTrail {
         return acc;
       }, {} as Record<ExcalidrawElement["id"], true>);
 
+      if (this.keepPreviousSelection) {
+        for (const id of Object.keys(prevState.selectedElementIds)) {
+          nextSelectedElementIds[id] = true;
+        }
+      }
+
       for (const [id] of Object.entries(nextSelectedElementIds)) {
         const element = this.app.scene.getNonDeletedElement(id);
         if (element && isFrameLikeElement(element)) {
@@ -123,9 +140,11 @@ export class LassoTrail extends AnimatedTrail {
     });
   };
 
-  addPointToPath = (x: number, y: number) => {
+  addPointToPath = (x: number, y: number, keepPreviousSelection = false) => {
     super.addPointToPath(x, y);
 
+    this.keepPreviousSelection = keepPreviousSelection;
+
     this.app.setState({
       lassoSelection: {
         points: