Don't split first_move while homing or probing

While homing or probing it might be bad if the stop/trobe triggers during the first part and the second is still in the buffer.

Marlin/Marlin_main.cpp

@@ -1719,6 +1719,7 @@ static void setup_for_endstop_or_probe_move() {
   saved_feedrate_percentage = feedrate_percentage;
   feedrate_percentage = 100;
   refresh_cmd_timeout();
+  planner.split_first_move = false; 
 }
 
 static void clean_up_after_endstop_or_probe_move() {
@@ -1728,6 +1729,7 @@ static void clean_up_after_endstop_or_probe_move() {
   feedrate_mm_s = saved_feedrate_mm_s;
   feedrate_percentage = saved_feedrate_percentage;
   refresh_cmd_timeout();
+  planner.split_first_move = true; 
 }
 
 #if HAS_BED_PROBE

Marlin/planner.cpp

@@ -92,6 +92,8 @@ float Planner::max_feedrate_mm_s[XYZE_N], // Max speeds in mm per second
   uint8_t Planner::last_extruder = 0;     // Respond to extruder change
 #endif
 
+bool Planner::split_first_move = true;
+
 int16_t Planner::flow_percentage[EXTRUDERS] = ARRAY_BY_EXTRUDERS1(100); // Extrusion factor for each extruder
 
 float Planner::e_factor[EXTRUDERS],               // The flow percentage and volumetric multiplier combine to scale E movement
@@ -1433,8 +1435,8 @@ void Planner::_buffer_line(const float &a, const float &b, const float &c, const
   if (DEBUGGING(DRYRUN))
     position[E_AXIS] = target[E_AXIS];
 
-  // Always split the first move into one longer and one shorter move
+  // Always split the first move into two (if not homing or probing)
-  if (!blocks_queued()) {
+  if (!blocks_queued() && split_first_move) {
     #define _BETWEEN(A) (position[A##_AXIS] + target[A##_AXIS]) >> 1
     const int32_t between[XYZE] = { _BETWEEN(X), _BETWEEN(Y), _BETWEEN(Z), _BETWEEN(E) };
     DISABLE_STEPPER_DRIVER_INTERRUPT();

Marlin/planner.h

@@ -161,6 +161,7 @@ class Planner {
                  travel_acceleration,  // Travel acceleration mm/s^2  DEFAULT ACCELERATION for all NON printing moves. M204 MXXXX
                  max_jerk[XYZE],       // The largest speed change requiring no acceleration
                  min_travel_feedrate_mm_s;
+    static bool split_first_move;
 
     #if HAS_LEVELING
       static bool leveling_active;          // Flag that bed leveling is enabled