Bugfix

- Buffer flush added to the the end of M190 loop
- Inverted position polling when canceling print via display

Marlin/Marlin_main.cpp

@@ -8706,6 +8706,9 @@ inline void gcode_M109() {
     #if DISABLED(BUSY_WHILE_HEATING)
       KEEPALIVE_STATE(IN_HANDLER);
     #endif
+    
+     // flush the serial buffer after heating to prevent lockup by m105
+     flush_and_request_resend();
   }
 
 #endif // HAS_HEATED_BED