Position adjustment for bed leveling

Marlin/Marlin_main.cpp

@@ -6684,6 +6684,35 @@ inline void gcode_T(uint8_t tmp_extruder) {
         }
         // No extra case for AUTO_BED_LEVELING_FEATURE in DUAL_X_CARRIAGE. Does that mean they don't work together?
       #else // !DUAL_X_CARRIAGE
+
+        //
+        // Set current_position to the position of the new nozzle.
+        // Offsets are based on linear distance, so we need to get
+        // the resulting position in coordinate space.
+        //
+        // - With grid or 3-point leveling, offset XYZ by a tilted vector
+        // - With mesh leveling, update Z for the new position
+        // - Otherwise, just use the raw linear distance
+        //
+        // Software endstops are altered here too. Consider a case where:
+        //   E0 at X=0 ... E1 at X=10
+        // When we switch to E1 now X=10, but E1 can't move left.
+        // To express this we apply the change in XY to the software endstops.
+        // E1 can move farther right than E0, so the right limit is extended.
+        //
+        // Note that we don't adjust the Z software endstops. Why not?
+        // Consider a case where Z=0 (here) and switching to E1 makes Z=1
+        // because the bed is 1mm lower at the new position. As long as
+        // the first nozzle is out of the way, the carriage should be
+        // allowed to move 1mm lower. This technically "breaks" the
+        // Z software endstop. But this is technically correct (and
+        // there is no viable alternative).
+        //
+        float xydiff[2] = {
+          hotend_offset[X_AXIS][tmp_extruder] - hotend_offset[X_AXIS][active_extruder],
+          hotend_offset[Y_AXIS][tmp_extruder] - hotend_offset[Y_AXIS][active_extruder]
+        };
+
         #if ENABLED(AUTO_BED_LEVELING_FEATURE)
           // Offset extruder, make sure to apply the bed level rotation matrix
           vector_3 tmp_offset_vec = vector_3(hotend_offset[X_AXIS][tmp_extruder],
@@ -6731,6 +6760,17 @@ inline void gcode_T(uint8_t tmp_extruder) {
 
         #endif // !AUTO_BED_LEVELING_FEATURE
 
+          // The newly-selected extruder XY is actually at...
+          current_position[X_AXIS] += xydiff[X_AXIS];
+          current_position[Y_AXIS] += xydiff[Y_AXIS];
+
+        #endif // no bed leveling
+
+        for (uint8_t i = X_AXIS; i <= Y_AXIS; i++) {
+          position_shift[i] += xydiff[i];
+          update_software_endstops((AxisEnum)i);
+        }
+
         // Set the new active extruder
         active_extruder = tmp_extruder;