"preserve unchanged props" will be automatically checked when editing attributes on multiple object selection and unchecked on single object selection as this is the 99.9% use case

src/editprop.c

@@ -1501,6 +1501,13 @@ void edit_property(int x)
    return;
  } /* if((xctx->lastsel==0 ) */
 
+ /* set 'preserve unchanged properties if multiple selection */
+ if(xctx->lastsel > 1) {
+   tclsetvar("preserve_unchanged_attrs", "1");
+ } else {
+   tclsetvar("preserve_unchanged_attrs", "0");
+ }
+
  switch(xctx->sel_array[0].type)
  {
   case ELEMENT:

src/save.c

@@ -124,7 +124,8 @@ int filter_data(const char *din,  const size_t ilen,
       fprintf(stderr, "error: conversion failed\n");
       ret = 1;
     }
-    _exit(ret);
+    _exit(ret); /* childs should always use _exit() to avoid 
+                 * flushing open stdio streams and other unwanted side effects */
   }
   /* parent */
   close(p1[0]); /*only write to p1 */