Fix: Junction Deviation calculates wrong cos(theta) on CoreXY (#12879)

Marlin/src/module/planner.cpp

@@ -2392,6 +2392,15 @@ bool Planner::_populate_block(block_t * const block, bool split_move,
       };
     #endif
 
+    #if IS_CORE && ENABLED(JUNCTION_DEVIATION)
+      /**
+       * On CoreXY the length of the vector [A,B] is SQRT(2) times the length of the head movement vector [X,Y].
+       * So taking Z and E into account, we cannot scale to a unit vector with "inverse_millimeters".
+       * => normalize the complete junction vector
+       */
+      normalize_junction_vector(unit_vec);
+    #endif
+
     // Skip first block or when previous_nominal_speed is used as a flag for homing and offset cycles.
     if (moves_queued && !UNEAR_ZERO(previous_nominal_speed_sqr)) {
       // Compute cosine of angle between previous and current path. (prev_unit_vec is negative)