screen: make update_screen() the only entry point for redrawing

update_single_line() was only used for 'concealcursor'. But 'cursorline'
has very similiar characteristics (redraw both lines on move cursor
between lines) and works without its own special entry point to the
redraw subsystem.

Later on 'concealcursor' and 'cursorline' could share more logic, but for
now make the former use standard redrawWinline(). Make sure it is called
before update_screen(), so that it is immediately visible.

Get rid of update_prepare() and update_finish(), and all issues from
them and their callsites not being in sync with changes to
update_screen()

src/nvim/edit.c

@@ -1452,21 +1452,24 @@ ins_redraw (
     }
   }
 
-  if (must_redraw)
-    update_screen(0);
-  else if (clear_cmdline || redraw_cmdline)
-    showmode();                 /* clear cmdline and show mode */
   if ((conceal_update_lines
        && (conceal_old_cursor_line != conceal_new_cursor_line
            || conceal_cursor_line(curwin)))
       || need_cursor_line_redraw) {
-    if (conceal_old_cursor_line != conceal_new_cursor_line)
-      update_single_line(curwin, conceal_old_cursor_line);
-    update_single_line(curwin, conceal_new_cursor_line == 0
-        ? curwin->w_cursor.lnum : conceal_new_cursor_line);
+    if (conceal_old_cursor_line != conceal_new_cursor_line) {
+      redrawWinline(curwin, conceal_old_cursor_line);
+    }
+    redrawWinline(curwin, conceal_new_cursor_line == 0
+                  ? curwin->w_cursor.lnum : conceal_new_cursor_line);
     curwin->w_valid &= ~VALID_CROW;
   }
-  showruler(FALSE);
+
+  if (must_redraw) {
+    update_screen(0);
+  } else if (clear_cmdline || redraw_cmdline) {
+    showmode();  // clear cmdline and show mode
+  }
+  showruler(false);
   setcursor();
   emsg_on_display = FALSE;      /* may remove error message now */
 }

src/nvim/normal.c

@@ -1246,6 +1246,25 @@ static void normal_redraw(NormalState *s)
   update_topline();
   validate_cursor();
 
+  // TODO(bfredl): this logic is only used for 'concealcursor', not
+  // 'cursorline'. Maybe we can eliminate this check (and in edit.c) by
+  // checking for 'concealcursor' wherever we check for 'cursorline'
+  if (s->conceal_update_lines
+      && (s->conceal_old_cursor_line !=
+        s->conceal_new_cursor_line
+        || conceal_cursor_line(curwin)
+        || need_cursor_line_redraw)) {
+    if (s->conceal_old_cursor_line !=
+        s->conceal_new_cursor_line
+        && s->conceal_old_cursor_line <=
+        curbuf->b_ml.ml_line_count) {
+      redrawWinline(curwin, s->conceal_old_cursor_line);
+    }
+
+    redrawWinline(curwin, s->conceal_new_cursor_line);
+    curwin->w_valid &= ~VALID_CROW;
+  }
+
   if (VIsual_active) {
     update_curbuf(INVERTED);  // update inverted part
   } else if (must_redraw) {
@@ -1281,22 +1300,6 @@ static void normal_redraw(NormalState *s)
   may_clear_sb_text();  // clear scroll-back text on next msg
   showruler(false);
 
-  if (s->conceal_update_lines
-      && (s->conceal_old_cursor_line !=
-        s->conceal_new_cursor_line
-        || conceal_cursor_line(curwin)
-        || need_cursor_line_redraw)) {
-    if (s->conceal_old_cursor_line !=
-        s->conceal_new_cursor_line
-        && s->conceal_old_cursor_line <=
-        curbuf->b_ml.ml_line_count) {
-      update_single_line(curwin, s->conceal_old_cursor_line);
-    }
-
-    update_single_line(curwin, s->conceal_new_cursor_line);
-    curwin->w_valid &= ~VALID_CROW;
-  }
-
   setcursor();
 }
 
@@ -7089,7 +7092,7 @@ static void n_opencmd(cmdarg_T *cap)
                      ? OPENLINE_DO_COM : 0,
                      0)) {
       if (curwin->w_p_cole > 0 && oldline != curwin->w_cursor.lnum) {
-        update_single_line(curwin, oldline);
+        redrawWinline(curwin, oldline);
       }
       if (curwin->w_p_cul) {
         // force redraw of cursorline

src/nvim/screen.c

@@ -482,25 +482,6 @@ void update_screen(int type)
 
 }
 
-// Prepare for updating one or more windows.
-// Caller must check for "updating_screen" already set to avoid recursiveness.
-static void update_prepare(void)
-{
-  updating_screen = true;
-  start_search_hl();
-}
-
-// Finish updating one or more windows.
-static void update_finish(void)
-{
-  if (redraw_cmdline) {
-    showmode();
-  }
-
-  end_search_hl();
-  updating_screen = false;
-}
-
 /*
  * Return TRUE if the cursor line in window "wp" may be concealed, according
  * to the 'concealcursor' option.
@@ -537,39 +518,6 @@ void conceal_check_cursor_line(void)
   }
 }
 
-void update_single_line(win_T *wp, linenr_T lnum)
-{
-  int row;
-  int j;
-
-  // Don't do anything if the screen structures are (not yet) valid.
-  if (linebuf_char == NULL || updating_screen) {
-    return;
-  }
-
-  if (lnum >= wp->w_topline && lnum < wp->w_botline
-      && foldedCount(wp, lnum, &win_foldinfo) == 0) {
-    update_prepare();
-
-    row = 0;
-    for (j = 0; j < wp->w_lines_valid; ++j) {
-      if (lnum == wp->w_lines[j].wl_lnum) {
-        init_search_hl(wp);
-        prepare_search_hl(wp, lnum);
-        update_window_hl(wp, false);
-        // allocate window grid if not already
-        win_grid_alloc(wp);
-        win_line(wp, lnum, row, row + wp->w_lines[j].wl_size, false, false);
-        break;
-      }
-      row += wp->w_lines[j].wl_size;
-    }
-
-    update_finish();
-  }
-  need_cursor_line_redraw = false;
-}
-
 
 /*
  * Update a single window.

src/nvim/window.c

@@ -3542,11 +3542,13 @@ void win_goto(win_T *wp)
 
   win_enter(wp, true);
 
-  /* Conceal cursor line in previous window, unconceal in current window. */
-  if (win_valid(owp) && owp->w_p_cole > 0 && !msg_scrolled)
-    update_single_line(owp, owp->w_cursor.lnum);
-  if (curwin->w_p_cole > 0 && !msg_scrolled)
-    need_cursor_line_redraw = TRUE;
+  // Conceal cursor line in previous window, unconceal in current window.
+  if (win_valid(owp) && owp->w_p_cole > 0 && !msg_scrolled) {
+    redrawWinline(owp, owp->w_cursor.lnum);
+  }
+  if (curwin->w_p_cole > 0 && !msg_scrolled) {
+    need_cursor_line_redraw = true;
+  }
 }