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

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;