softwrap: iterate through softwrapped chunks in adjust_viewport()

Use go_back_chunks() to adjust edittop, instead of special casing
the computation of goal when softwrapping.  Now softwrap mode and
non-softwrap mode will behave the same way when edittop can be
partially scrolled off the screen, which is forthcoming.

(Note that the top line of the screen can't be partially scrolled
yet, so we have to work around that for now.)

src/winio.c

@@ -3072,6 +3072,10 @@ void adjust_viewport(update_type manner)
 {
     int goal = 0;
 
+    /* FIXME: This should be replaced with openfile->firstcolumn when the
+     * latter is added. */
+    size_t firstcolumn = 0;
+
     /* If manner is CENTERING, move edittop half the number of window rows
      * back from current.  If manner is FLOWING, move edittop back 0 rows
      * or (editwinrows - 1) rows, depending on where current has moved.
@@ -3083,13 +3087,8 @@ void adjust_viewport(update_type manner)
     if (manner == CENTERING)
 	goal = editwinrows / 2;
     else if (manner == FLOWING) {
-	if (!current_is_above_screen()) {
+	if (!current_is_above_screen())
 	    goal = editwinrows - 1;
-#ifndef NANO_TINY
-	    if (ISSET(SOFTWRAP))
-		goal -= strlenpt(openfile->current->data) / editwincols;
-#endif
-	}
     } else {
 	goal = openfile->current_y;
 
@@ -3100,17 +3099,14 @@ void adjust_viewport(update_type manner)
 
     openfile->edittop = openfile->current;
 
-    while (goal > 0 && openfile->edittop->prev != NULL) {
-	openfile->edittop = openfile->edittop->prev;
-	goal--;
 #ifndef NANO_TINY
-	if (ISSET(SOFTWRAP)) {
-	    goal -= strlenpt(openfile->edittop->data) / editwincols;
-	    if (goal < 0)
-		openfile->edittop = openfile->edittop->next;
-	}
+    if (ISSET(SOFTWRAP))
+	firstcolumn = (xplustabs() / editwincols) * editwincols;
 #endif
-    }
+
+    /* Move edittop back goal rows, starting at current[current_x]. */
+    go_back_chunks(goal, &openfile->edittop, &firstcolumn);
+
 #ifdef DEBUG
     fprintf(stderr, "adjust_viewport(): setting edittop to lineno %ld\n", (long)openfile->edittop->lineno);
 #endif