spelling: don't partition the file for replacements in a marked region

Partitioning the file makes the undo system lose track, so that undoing
things has a good chance of losing data.  Instead, just make sure that
the region is marked "backwards", with current at the top and the mark
at the bottom, and then let the replace loop take care of not going
outside of the marked region.

This also has the effect that if the marked region fits entirely on the
screen, or all the misspellings are onscreen, then the screen will not
be scrolled at all.  Which makes for a smooth experience.

This fixes https://savannah.gnu.org/bugs/?47836,
and fixes https://savannah.gnu.org/bugs/?45573.

src/search.c

@@ -281,6 +281,9 @@ int findnextstr(
 
     enable_nodelay();
 
+    if (begin == NULL)
+	came_full_circle = FALSE;
+
     /* Start searching through the lines, looking for the needle. */
     while (TRUE) {
 	/* Glance at the keyboard once every second. */
@@ -323,7 +326,7 @@ int findnextstr(
 	}
 
 	/* If we're back at the beginning, then there is no needle. */
-	if (came_full_circle && begin != NULL) {
+	if (came_full_circle) {
 	    not_found_msg(needle);
 	    disable_nodelay();
 	    return 0;

src/text.c

@@ -2364,8 +2364,6 @@ bool do_int_spell_fix(const char *word)
 	/* A storage place for the current flag settings. */
 #ifndef NANO_TINY
     bool old_mark_set = openfile->mark_set;
-    bool added_magicline = FALSE;
-	/* Whether we added a magicline after filebot. */
     bool right_side_up = FALSE;
 	/* TRUE if (mark_begin, mark_begin_x) is the top of the mark,
 	 * FALSE if (current, current_x) is. */
@@ -2393,25 +2391,27 @@ bool do_int_spell_fix(const char *word)
     last_search = mallocstrcpy(NULL, word);
 
 #ifndef NANO_TINY
+    /* If the mark is on, start at the beginning of the marked region. */
     if (old_mark_set) {
-	/* Trim the filestruct so that it contains only the marked text. */
 	mark_order((const filestruct **)&top, &top_x,
 	    (const filestruct **)&bot, &bot_x, &right_side_up);
-	filepart = partition_filestruct(top, top_x, bot, bot_x);
-
-	/* Foresay whether spell correction will add a magicline. */
-	if (!ISSET(NO_NEWLINES))
-	    added_magicline = (openfile->filebot->data[0] != '\0');
-
-	/* Turn the mark off. */
+	/* If the region is marked normally, swap the end points, so that
+	 * (current, current_x) (where searching starts) is at the top. */
+	if (right_side_up) {
+	    openfile->current = top;
+	    openfile->current_x = (size_t)(top_x - 1);
+	    openfile->mark_begin = bot;
+	    openfile->mark_begin_x = bot_x;
+	}
 	openfile->mark_set = FALSE;
-    }
+    } else
 #endif
-
-    /* Start from the top of the file. */
-    openfile->edittop = openfile->fileage;
-    openfile->current = openfile->fileage;
-    openfile->current_x = (size_t)-1;
+    /* Otherwise, start from the top of the file. */
+    {
+	openfile->edittop = openfile->fileage;
+	openfile->current = openfile->fileage;
+	openfile->current_x = (size_t)-1;
+    }
 
     /* Find the first whole occurrence of word. */
     result = findnextstr(TRUE, NULL, 0, word, NULL);
@@ -2445,43 +2445,41 @@ bool do_int_spell_fix(const char *word)
 	/* If a replacement was given, go through all occurrences. */
 	if (proceed && strcmp(word, answer) != 0) {
 	    openfile->current_x--;
+#ifndef NANO_TINY
+	    /* Replacements should happen only in the marked region. */
+	    openfile->mark_set = old_mark_set;
+#endif
 	    do_replace_loop(TRUE, current_save, &current_x_save, word);
 	}
     }
 
 #ifndef NANO_TINY
     if (old_mark_set) {
-	/* If a magicline was added, remove it again. */
-	if (added_magicline)
-	    remove_magicline();
-
-	/* Put the beginning and the end of the mark at the beginning
-	 * and the end of the spell-checked text. */
-	if (openfile->fileage == openfile->filebot)
-	    bot_x += top_x;
+	/* Restore the (compensated) end points of the marked region. */
 	if (right_side_up) {
+	    openfile->current = openfile->mark_begin;
+	    openfile->current_x = openfile->mark_begin_x;
+	    openfile->mark_begin = top;
 	    openfile->mark_begin_x = top_x;
-	    current_x_save = bot_x;
 	} else {
-	    current_x_save = top_x;
-	    openfile->mark_begin_x = bot_x;
+	    openfile->current = top;
+	    openfile->current_x = top_x;
 	}
-
-	/* Unpartition the filestruct so that it contains all the text
-	 * again, and turn the mark back on. */
-	unpartition_filestruct(&filepart);
 	openfile->mark_set = TRUE;
+    } else
+#endif
+    {
+	/* Restore the (compensated) cursor position. */
+	openfile->current = current_save;
+	openfile->current_x = current_x_save;
     }
-#endif /* !NANO_TINY */
 
     /* Restore the string that was last searched for. */
     free(last_search);
     last_search = save_search;
 
-    /* Restore where we were. */
+    /* Restore the viewport to where it was. */
     openfile->edittop = edittop_save;
-    openfile->current = current_save;
-    openfile->current_x = current_x_save;
 
     /* Restore the settings of the global flags. */
     memcpy(flags, stash, sizeof(flags));