[2.0.0] Better off mesh behavior for UBL (#8714)

* Better off mesh behaviour for UBL

* Suggested ubl.h changes

Marlin/src/config/examples/gCreate/gMax1.5+/Configuration.h

@@ -973,7 +973,7 @@
   //========================= Unified Bed Leveling ============================
   //===========================================================================
 
-  //#define MESH_EDIT_GFX_OVERLAY   // Display a graphics overlay while editing the mesh
+  #define MESH_EDIT_GFX_OVERLAY     // Display a graphics overlay while editing the mesh
 
   #define MESH_INSET 45             // Mesh inset margin on print area
   #define GRID_MAX_POINTS_X 10      // Don't use more than 15 points per axis, implementation limited.

Marlin/src/feature/bedlevel/ubl/ubl.h

@@ -200,10 +200,10 @@ class unified_bed_leveling {
      * the case where the printer is making a vertical line that only crosses horizontal mesh lines.
      */
     inline static float z_correction_for_x_on_horizontal_mesh_line(const float &rx0, const int x1_i, const int yi) {
-      if (!WITHIN(x1_i, 0, GRID_MAX_POINTS_X - 2) || !WITHIN(yi, 0, GRID_MAX_POINTS_Y - 1)) {
+      if (!WITHIN(x1_i, 0, GRID_MAX_POINTS_X - 1) || !WITHIN(yi, 0, GRID_MAX_POINTS_Y - 1)) {
         #if ENABLED(DEBUG_LEVELING_FEATURE)
           if (DEBUGGING(LEVELING)) {
-            serialprintPGM( !WITHIN(x1_i, 0, GRID_MAX_POINTS_X - 1) ? PSTR("x1l_i") : PSTR("yi") );
+            serialprintPGM( !WITHIN(x1_i, 0, GRID_MAX_POINTS_X - 1) ? PSTR("x1_i") : PSTR("yi") );
             SERIAL_ECHOPAIR(" out of bounds in z_correction_for_x_on_horizontal_mesh_line(rx0=", rx0);
             SERIAL_ECHOPAIR(",x1_i=", x1_i);
             SERIAL_ECHOPAIR(",yi=", yi);
@@ -217,17 +217,19 @@ class unified_bed_leveling {
       const float xratio = (rx0 - mesh_index_to_xpos(x1_i)) * (1.0 / (MESH_X_DIST)),
                   z1 = z_values[x1_i][yi];
 
-      return z1 + xratio * (z_values[x1_i + 1][yi] - z1);
+      return z1 + xratio * (z_values[min(x1_i, GRID_MAX_POINTS_X - 2) + 1][yi] - z1);  // Don't allow x1_i+1 to be past the end of the array
+                                                                                       // If it is, it is clamped to the last element of the 
+                                                                                       // z_values[][] array and no correction is applied.
     }
 
     //
     // See comments above for z_correction_for_x_on_horizontal_mesh_line
     //
     inline static float z_correction_for_y_on_vertical_mesh_line(const float &ry0, const int xi, const int y1_i) {
-      if (!WITHIN(xi, 0, GRID_MAX_POINTS_X - 1) || !WITHIN(y1_i, 0, GRID_MAX_POINTS_Y - 2)) {
+      if (!WITHIN(xi, 0, GRID_MAX_POINTS_X - 1) || !WITHIN(y1_i, 0, GRID_MAX_POINTS_Y - 1)) {
         #if ENABLED(DEBUG_LEVELING_FEATURE)
           if (DEBUGGING(LEVELING)) {
-            serialprintPGM( !WITHIN(xi, 0, GRID_MAX_POINTS_X - 1) ? PSTR("xi") : PSTR("yl_i") );
+            serialprintPGM( !WITHIN(xi, 0, GRID_MAX_POINTS_X - 1) ? PSTR("xi") : PSTR("y1_i") );
             SERIAL_ECHOPAIR(" out of bounds in z_correction_for_y_on_vertical_mesh_line(ry0=", ry0);
             SERIAL_ECHOPAIR(", xi=", xi);
             SERIAL_ECHOPAIR(", y1_i=", y1_i);
@@ -241,7 +243,9 @@ class unified_bed_leveling {
       const float yratio = (ry0 - mesh_index_to_ypos(y1_i)) * (1.0 / (MESH_Y_DIST)),
                   z1 = z_values[xi][y1_i];
 
-      return z1 + yratio * (z_values[xi][y1_i + 1] - z1);
+      return z1 + yratio * (z_values[xi][min(y1_i, GRID_MAX_POINTS_Y - 2) + 1] - z1);  // Don't allow y1_i+1 to be past the end of the array
+                                                                                       // If it is, it is clamped to the last element of the 
+                                                                                       // z_values[][] array and no correction is applied.
     }
 
     /**
@@ -252,29 +256,15 @@ class unified_bed_leveling {
      */
     static float get_z_correction(const float &rx0, const float &ry0) {
       const int8_t cx = get_cell_index_x(rx0),
-                   cy = get_cell_index_y(ry0);
-
-      if (!WITHIN(cx, 0, GRID_MAX_POINTS_X - 2) || !WITHIN(cy, 0, GRID_MAX_POINTS_Y - 2)) {
-
-        SERIAL_ECHOPAIR("? in get_z_correction(rx0=", rx0);
-        SERIAL_ECHOPAIR(", ry0=", ry0);
-        SERIAL_CHAR(')');
-        SERIAL_EOL();
-
-        #if ENABLED(ULTRA_LCD)
-          strcpy(lcd_status_message, "get_z_correction() indexes out of range.");
-          lcd_quick_feedback();
-        #endif
-        return NAN;
-      }
+                   cy = get_cell_index_y(ry0); // return values are clamped
 
       const float z1 = calc_z0(rx0,
                                mesh_index_to_xpos(cx), z_values[cx][cy],
-                               mesh_index_to_xpos(cx + 1), z_values[cx + 1][cy]);
+                               mesh_index_to_xpos(cx + 1), z_values[min(cx, GRID_MAX_POINTS_X - 2) + 1][cy]);
 
       const float z2 = calc_z0(rx0,
-                               mesh_index_to_xpos(cx), z_values[cx][cy + 1],
-                               mesh_index_to_xpos(cx + 1), z_values[cx + 1][cy + 1]);
+                               mesh_index_to_xpos(cx), z_values[cx][min(cy, GRID_MAX_POINTS_Y - 2) + 1],
+                               mesh_index_to_xpos(cx + 1), z_values[min(cx, GRID_MAX_POINTS_X - 2) + 1][min(cy, GRID_MAX_POINTS_Y - 2) + 1]);
 
       float z0 = calc_z0(ry0,
                          mesh_index_to_ypos(cy), z1,