vim-patch:9.0.0901: setting w_leftcol and handling side effects is confusing

Problem:    Setting w_leftcol and handling side effects is confusing.
Solution:   Use a function to set w_leftcol() and handle side effects.

0c34d56264

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/cursor.c

@@ -439,23 +439,27 @@ void adjust_cursor_col(void)
   }
 }
 
-/// When curwin->w_leftcol has changed, adjust the cursor position.
+/// Set "curwin->w_leftcol" to "leftcol".
+/// Adjust the cursor position if needed.
 ///
 /// @return  true if the cursor was moved.
-bool leftcol_changed(void)
+bool set_leftcol(colnr_T leftcol)
 {
-  // TODO(hinidu): I think it should be colnr_T or int, but p_siso is long.
-  // Perhaps we can change p_siso to int.
-  int64_t lastcol;
-  colnr_T s, e;
-  bool retval = false;
+  // Return quickly when there is no change.
+  if (curwin->w_leftcol == leftcol) {
+    return false;
+  }
+  curwin->w_leftcol = leftcol;
 
   changed_cline_bef_curs();
-  lastcol = curwin->w_leftcol + curwin->w_width_inner - curwin_col_off() - 1;
+  // TODO(hinidu): I think it should be colnr_T or int, but p_siso is long.
+  // Perhaps we can change p_siso to int.
+  int64_t lastcol = curwin->w_leftcol + curwin->w_width_inner - curwin_col_off() - 1;
   validate_virtcol();
 
+  bool retval = false;
   // If the cursor is right or left of the screen, move it to last or first
-  // character.
+  // visible character.
   long siso = get_sidescrolloff_value(curwin);
   if (curwin->w_virtcol > (colnr_T)(lastcol - siso)) {
     retval = true;
@@ -468,6 +472,7 @@ bool leftcol_changed(void)
   // If the start of the character under the cursor is not on the screen,
   // advance the cursor one more char.  If this fails (last char of the
   // line) adjust the scrolling.
+  colnr_T s, e;
   getvvcol(curwin, &curwin->w_cursor, &s, NULL, &e);
   if (e > (colnr_T)lastcol) {
     retval = true;

src/nvim/mouse.c

@@ -1648,8 +1648,6 @@ bool mouse_scroll_horiz(int dir)
     return false;
   }
 
-  curwin->w_leftcol = (colnr_T)leftcol;
-
   // When the line of the cursor is too short, move the cursor to the
   // longest visible line.
   if (!virtual_active()
@@ -1658,7 +1656,7 @@ bool mouse_scroll_horiz(int dir)
     curwin->w_cursor.col = 0;
   }
 
-  return leftcol_changed();
+  return set_leftcol(leftcol);
 }
 
 /// Adjusts the clicked column position when 'conceallevel' > 0

src/nvim/normal.c

@@ -2158,9 +2158,8 @@ void check_scrollbind(linenr_T topline_diff, long leftcol_diff)
     }
 
     // do the horizontal scroll
-    if (want_hor && curwin->w_leftcol != tgt_leftcol) {
-      curwin->w_leftcol = tgt_leftcol;
-      leftcol_changed();
+    if (want_hor) {
+      (void)set_leftcol(tgt_leftcol);
     }
   }
 
@@ -2643,7 +2642,7 @@ void scroll_redraw(int up, long count)
                scrollup(count, true) :
                scrolldown(count, true);
 
-  if (get_scrolloff_value(curwin)) {
+  if (get_scrolloff_value(curwin) > 0) {
     // Adjust the cursor position for 'scrolloff'.  Mark w_topline as
     // valid, otherwise the screen jumps back at the end of the file.
     cursor_correct();
@@ -2894,27 +2893,21 @@ static void nv_zet(cmdarg_T *cap)
   case 'h':
   case K_LEFT:
     if (!curwin->w_p_wrap) {
-      if ((colnr_T)cap->count1 > curwin->w_leftcol) {
-        curwin->w_leftcol = 0;
-      } else {
-        curwin->w_leftcol -= (colnr_T)cap->count1;
-      }
-      leftcol_changed();
+      (void)set_leftcol((colnr_T)cap->count1 > curwin->w_leftcol
+                        ? 0 : curwin->w_leftcol - (colnr_T)cap->count1);
     }
     break;
 
-  // "zL" - scroll screen left half-page
+  // "zL" - scroll window left half-page
   case 'L':
     cap->count1 *= curwin->w_width_inner / 2;
     FALLTHROUGH;
 
-  // "zl" - scroll screen to the left
+  // "zl" - scroll window to the left if not wrapping
   case 'l':
   case K_RIGHT:
     if (!curwin->w_p_wrap) {
-      // scroll the window left
-      curwin->w_leftcol += (colnr_T)cap->count1;
-      leftcol_changed();
+      (void)set_leftcol(curwin->w_leftcol + (colnr_T)cap->count1);
     }
     break;