feat: Try to preserve line angle on SHIFT+drag (#9570)

packages/element/src/linearElementEditor.ts

@@ -149,6 +149,7 @@ export class LinearElementEditor {
   public readonly hoverPointIndex: number;
   public readonly segmentMidPointHoveredCoords: GlobalPoint | null;
   public readonly elbowed: boolean;
+  public readonly customLineAngle: number | null;
 
   constructor(
     element: NonDeleted<ExcalidrawLinearElement>,
@@ -186,6 +187,7 @@ export class LinearElementEditor {
     this.hoverPointIndex = -1;
     this.segmentMidPointHoveredCoords = null;
     this.elbowed = isElbowArrow(element) && element.elbowed;
+    this.customLineAngle = null;
   }
 
   // ---------------------------------------------------------------------------
@@ -289,6 +291,7 @@ export class LinearElementEditor {
     const { elementId } = linearElementEditor;
     const elementsMap = scene.getNonDeletedElementsMap();
     const element = LinearElementEditor.getElement(elementId, elementsMap);
+    let customLineAngle = linearElementEditor.customLineAngle;
     if (!element) {
       return null;
     }
@@ -329,6 +332,12 @@ export class LinearElementEditor {
         const selectedIndex = selectedPointsIndices[0];
         const referencePoint =
           element.points[selectedIndex === 0 ? 1 : selectedIndex - 1];
+        customLineAngle =
+          linearElementEditor.customLineAngle ??
+          Math.atan2(
+            element.points[selectedIndex][1] - referencePoint[1],
+            element.points[selectedIndex][0] - referencePoint[0],
+          );
 
         const [width, height] = LinearElementEditor._getShiftLockedDelta(
           element,
@@ -336,6 +345,7 @@ export class LinearElementEditor {
           referencePoint,
           pointFrom(scenePointerX, scenePointerY),
           event[KEYS.CTRL_OR_CMD] ? null : app.getEffectiveGridSize(),
+          customLineAngle,
         );
 
         LinearElementEditor.movePoints(
@@ -457,6 +467,7 @@ export class LinearElementEditor {
             ? lastClickedPoint
             : -1,
         isDragging: true,
+        customLineAngle,
       };
     }
 
@@ -574,6 +585,7 @@ export class LinearElementEditor {
           : selectedPointsIndices,
       isDragging: false,
       pointerOffset: { x: 0, y: 0 },
+      customLineAngle: null,
     };
   }
 
@@ -1595,6 +1607,7 @@ export class LinearElementEditor {
     referencePoint: LocalPoint,
     scenePointer: GlobalPoint,
     gridSize: NullableGridSize,
+    customLineAngle?: number,
   ) {
     const referencePointCoords = LinearElementEditor.getPointGlobalCoordinates(
       element,
@@ -1620,6 +1633,7 @@ export class LinearElementEditor {
       referencePointCoords[1],
       gridX,
       gridY,
+      customLineAngle,
     );
 
     return pointRotateRads(

packages/element/src/sizeHelpers.ts

@@ -2,6 +2,12 @@ import {
   SHIFT_LOCKING_ANGLE,
   viewportCoordsToSceneCoords,
 } from "@excalidraw/common";
+import {
+  normalizeRadians,
+  radiansBetweenAngles,
+  radiansDifference,
+  type Radians,
+} from "@excalidraw/math";
 
 import { pointsEqual } from "@excalidraw/math";
 
@@ -152,13 +158,42 @@ export const getLockedLinearCursorAlignSize = (
   originY: number,
   x: number,
   y: number,
+  customAngle?: number,
 ) => {
   let width = x - originX;
   let height = y - originY;
 
-  const lockedAngle =
-    Math.round(Math.atan(height / width) / SHIFT_LOCKING_ANGLE) *
-    SHIFT_LOCKING_ANGLE;
+  const angle = Math.atan2(height, width) as Radians;
+  let lockedAngle = (Math.round(angle / SHIFT_LOCKING_ANGLE) *
+    SHIFT_LOCKING_ANGLE) as Radians;
+
+  if (customAngle) {
+    // If custom angle is provided, we check if the angle is close to the
+    // custom angle, snap to that if close engough, otherwise snap to the
+    // higher or lower angle depending on the current angle vs custom angle.
+    const lower = (Math.floor(customAngle / SHIFT_LOCKING_ANGLE) *
+      SHIFT_LOCKING_ANGLE) as Radians;
+    if (
+      radiansBetweenAngles(
+        angle,
+        lower,
+        (lower + SHIFT_LOCKING_ANGLE) as Radians,
+      )
+    ) {
+      if (
+        radiansDifference(angle, customAngle as Radians) <
+        SHIFT_LOCKING_ANGLE / 6
+      ) {
+        lockedAngle = customAngle as Radians;
+      } else if (
+        normalizeRadians(angle) > normalizeRadians(customAngle as Radians)
+      ) {
+        lockedAngle = (lower + SHIFT_LOCKING_ANGLE) as Radians;
+      } else {
+        lockedAngle = lower;
+      }
+    }
+  }
 
   if (lockedAngle === 0) {
     height = 0;

packages/element/tests/linearElementEditor.test.tsx

@@ -1411,5 +1411,55 @@ describe("Test Linear Elements", () => {
       expect(line.points[line.points.length - 1][0]).toBe(20);
       expect(line.points[line.points.length - 1][1]).toBe(-20);
     });
+
+    it("should preserve original angle when dragging endpoint with SHIFT key", () => {
+      createTwoPointerLinearElement("line");
+      const line = h.elements[0] as ExcalidrawLinearElement;
+      enterLineEditingMode(line);
+
+      const elementsMap = arrayToMap(h.elements);
+      const points = LinearElementEditor.getPointsGlobalCoordinates(
+        line,
+        elementsMap,
+      );
+
+      // Calculate original angle between first and last point
+      const originalAngle = Math.atan2(
+        points[1][1] - points[0][1],
+        points[1][0] - points[0][0],
+      );
+
+      // Drag the second point (endpoint) with SHIFT key pressed
+      const startPoint = pointFrom<GlobalPoint>(points[1][0], points[1][1]);
+      const endPoint = pointFrom<GlobalPoint>(
+        startPoint[0] + 4,
+        startPoint[1] + 4,
+      );
+
+      // Perform drag with SHIFT key modifier
+      Keyboard.withModifierKeys({ shift: true }, () => {
+        mouse.downAt(startPoint[0], startPoint[1]);
+        mouse.moveTo(endPoint[0], endPoint[1]);
+        mouse.upAt(endPoint[0], endPoint[1]);
+      });
+
+      // Get updated points after drag
+      const updatedPoints = LinearElementEditor.getPointsGlobalCoordinates(
+        line,
+        elementsMap,
+      );
+
+      // Calculate new angle
+      const newAngle = Math.atan2(
+        updatedPoints[1][1] - updatedPoints[0][1],
+        updatedPoints[1][0] - updatedPoints[0][0],
+      );
+
+      // The angle should be preserved (within a small tolerance for floating point precision)
+      const angleDifference = Math.abs(newAngle - originalAngle);
+      const tolerance = 0.01; // Small tolerance for floating point precision
+
+      expect(angleDifference).toBeLessThan(tolerance);
+    });
   });
 });

packages/excalidraw/tests/__snapshots__/regressionTests.test.tsx.snap

@@ -8630,6 +8630,7 @@ exports[`regression tests > key 5 selects arrow tool > [end of test] appState 1`
   "selectedElementsAreBeingDragged": false,
   "selectedGroupIds": {},
   "selectedLinearElement": LinearElementEditor {
+    "customLineAngle": null,
     "elbowed": false,
     "elementId": "id0",
     "endBindingElement": "keep",
@@ -8853,6 +8854,7 @@ exports[`regression tests > key 6 selects line tool > [end of test] appState 1`]
   "selectedElementsAreBeingDragged": false,
   "selectedGroupIds": {},
   "selectedLinearElement": LinearElementEditor {
+    "customLineAngle": null,
     "elbowed": false,
     "elementId": "id0",
     "endBindingElement": "keep",
@@ -9270,6 +9272,7 @@ exports[`regression tests > key a selects arrow tool > [end of test] appState 1`
   "selectedElementsAreBeingDragged": false,
   "selectedGroupIds": {},
   "selectedLinearElement": LinearElementEditor {
+    "customLineAngle": null,
     "elbowed": false,
     "elementId": "id0",
     "endBindingElement": "keep",
@@ -9673,6 +9676,7 @@ exports[`regression tests > key l selects line tool > [end of test] appState 1`]
   "selectedElementsAreBeingDragged": false,
   "selectedGroupIds": {},
   "selectedLinearElement": LinearElementEditor {
+    "customLineAngle": null,
     "elbowed": false,
     "elementId": "id0",
     "endBindingElement": "keep",

packages/math/src/angle.ts

@@ -49,3 +49,35 @@ export function radiansToDegrees(degrees: Radians): Degrees {
 export function isRightAngleRads(rads: Radians): boolean {
   return Math.abs(Math.sin(2 * rads)) < PRECISION;
 }
+
+export function radiansBetweenAngles(
+  a: Radians,
+  min: Radians,
+  max: Radians,
+): boolean {
+  a = normalizeRadians(a);
+  min = normalizeRadians(min);
+  max = normalizeRadians(max);
+
+  if (min < max) {
+    return a >= min && a <= max;
+  }
+
+  // The range wraps around the 0 angle
+  return a >= min || a <= max;
+}
+
+export function radiansDifference(a: Radians, b: Radians): Radians {
+  a = normalizeRadians(a);
+  b = normalizeRadians(b);
+
+  let diff = a - b;
+
+  if (diff < -Math.PI) {
+    diff = (diff + 2 * Math.PI) as Radians;
+  } else if (diff > Math.PI) {
+    diff = (diff - 2 * Math.PI) as Radians;
+  }
+
+  return Math.abs(diff) as Radians;
+}