perf(statusline): UI elements are always redrawn on K_EVENT

Problem:    'statusline'-format UI elements are redrawn on each K_EVENT.
Solution:   Only redraw UI elements when something relevant has changed.

src/nvim/buffer_defs.h

@@ -1279,13 +1279,14 @@ struct window_S {
   bool w_redr_border;               // if true border must be redrawn
   bool w_redr_statuscol;            // if true 'statuscolumn' must be redrawn
 
-  // remember what is shown in the ruler for this window (if 'ruler' set)
+  // remember what is shown in the 'statusline'-format elements
-  pos_T w_ru_cursor;                // cursor position shown in ruler
+  pos_T w_stl_cursor;                // cursor position when last redrawn
-  colnr_T w_ru_virtcol;             // virtcol shown in ruler
+  colnr_T w_stl_virtcol;             // virtcol when last redrawn
-  linenr_T w_ru_topline;            // topline shown in ruler
+  linenr_T w_stl_topline;            // topline when last redrawn
-  linenr_T w_ru_line_count;         // line count used for ruler
+  linenr_T w_stl_line_count;         // line count when last redrawn
-  int w_ru_topfill;                 // topfill shown in ruler
+  int w_stl_topfill;                 // topfill when last redrawn
-  char w_ru_empty;                  // true if ruler shows 0-1 (empty line)
+  char w_stl_empty;                  // true if elements show 0-1 (empty line)
+  int w_stl_state;                   // State when last redrawn
 
   int w_alt_fnum;                   // alternate file (for # and CTRL-^)
 

src/nvim/drawscreen.c

@@ -92,6 +92,7 @@
 #include "nvim/profile.h"
 #include "nvim/regexp.h"
 #include "nvim/screen.h"
+#include "nvim/state.h"
 #include "nvim/statusline.h"
 #include "nvim/syntax.h"
 #include "nvim/terminal.h"
@@ -733,12 +734,28 @@ void show_cursor_info(bool always)
     return;
   }
 
+  int state = get_real_state();
+  int empty_line = (State & MODE_INSERT) == 0
+                   && *ml_get_buf(curwin->w_buffer, curwin->w_cursor.lnum, false) == NUL;
+
+  // Only draw when something changed.
+  validate_virtcol_win(curwin);
+  if (always
+      || curwin->w_cursor.lnum != curwin->w_stl_cursor.lnum
+      || curwin->w_cursor.col != curwin->w_stl_cursor.col
+      || curwin->w_virtcol != curwin->w_stl_virtcol
+      || curwin->w_cursor.coladd != curwin->w_stl_cursor.coladd
+      || curwin->w_topline != curwin->w_stl_topline
+      || curwin->w_buffer->b_ml.ml_line_count != curwin->w_stl_line_count
+      || curwin->w_topfill != curwin->w_stl_topfill
+      || empty_line != curwin->w_stl_empty
+      || state != curwin->w_stl_state) {
     win_check_ns_hl(curwin);
     if ((*p_stl != NUL || *curwin->w_p_stl != NUL)
         && (curwin->w_status_height || global_stl_height())) {
       redraw_custom_statusline(curwin);
     } else {
-    win_redr_ruler(curwin, always);
+      win_redr_ruler(curwin);
     }
     if (*p_wbr != NUL || *curwin->w_p_wbr != NUL) {
       win_redr_winbar(curwin);
@@ -757,6 +774,15 @@ void show_cursor_info(bool always)
     }
   }
 
+  curwin->w_stl_cursor = curwin->w_cursor;
+  curwin->w_stl_virtcol = curwin->w_virtcol;
+  curwin->w_stl_empty = (char)empty_line;
+  curwin->w_stl_topline = curwin->w_topline;
+  curwin->w_stl_line_count = curwin->w_buffer->b_ml.ml_line_count;
+  curwin->w_stl_topfill = curwin->w_topfill;
+  curwin->w_stl_state = state;
+}
+
 static void redraw_win_signcol(win_T *wp)
 {
   // If we can compute a change in the automatic sizing of the sign column

src/nvim/ex_getln.c

@@ -907,6 +907,9 @@ theend:
     ui_call_cmdline_hide(ccline.level);
     msg_ext_clear_later();
   }
+  if (!cmd_silent) {
+    status_redraw_all();  // redraw to show mode change
+  }
 
   cmdline_level--;
 

src/nvim/option.c

@@ -92,6 +92,7 @@
 #include "nvim/spell.h"
 #include "nvim/spellfile.h"
 #include "nvim/spellsuggest.h"
+#include "nvim/statusline.h"
 #include "nvim/strings.h"
 #include "nvim/tag.h"
 #include "nvim/terminal.h"
@@ -2251,8 +2252,13 @@ static char *set_bool_option(const int opt_idx, char *const varp, const int valu
     ui_call_option_set(cstr_as_string(options[opt_idx].fullname),
                        BOOLEAN_OBJ(*varp));
   }
-
+  if ((int *)varp == &p_ru || (int *)varp == &p_sc) {
-  comp_col();                       // in case 'ruler' or 'showcmd' changed
+    // in case 'ruler' or 'showcmd' changed
+    comp_col();
+    if ((int *)varp == &p_ru) {
+      win_redr_ruler(curwin);
+    }
+  }
   if (curwin->w_curswant != MAXCOL
       && (options[opt_idx].flags & (P_CURSWANT | P_RALL)) != 0) {
     curwin->w_set_curswant = true;

src/nvim/optionstr.c

@@ -49,6 +49,7 @@
 #include "nvim/spell.h"
 #include "nvim/spellfile.h"
 #include "nvim/spellsuggest.h"
+#include "nvim/statusline.h"
 #include "nvim/strings.h"
 #include "nvim/tag.h"
 #include "nvim/ui.h"
@@ -1220,6 +1221,7 @@ static void did_set_statusline(win_T *win, char **varp, char **gvarp, char **err
   }
   if (varp == &p_ruf && *errmsg == NULL) {
     comp_col();
+    win_redr_ruler(curwin);
   }
   // add / remove window bars for 'winbar'
   if (gvarp == &p_wbr) {

src/nvim/screen.c

@@ -611,7 +611,7 @@ int showmode(void)
   // the ruler is after the mode message and must be redrawn
   win_T *last = lastwin_nofloating();
   if (redrawing() && last->w_status_height == 0 && global_stl_height() == 0) {
-    win_redr_ruler(last, true);
+    win_redr_ruler(last);
   }
 
   redraw_cmdline = false;

src/nvim/statusline.c

@@ -162,7 +162,7 @@ void win_redr_status(win_T *wp)
                 (int)((size_t)this_ru_col - strlen(NameBuff) - 1), attr);
     }
 
-    win_redr_ruler(wp, true);
+    win_redr_ruler(wp);
 
     // Draw the 'showcmd' information if 'showcmdloc' == "statusline".
     if (p_sc && *p_sloc == 's') {
@@ -443,7 +443,7 @@ void win_redr_winbar(win_T *wp)
   entered = false;
 }
 
-void win_redr_ruler(win_T *wp, bool always)
+void win_redr_ruler(win_T *wp)
 {
   bool is_stl_global = global_stl_height() > 0;
   static bool did_show_ext_ruler = false;
@@ -473,23 +473,9 @@ void win_redr_ruler(win_T *wp, bool always)
   }
 
   // Check if not in Insert mode and the line is empty (will show "0-1").
-  int empty_line = false;
+  int empty_line = (State & MODE_INSERT) == 0
-  if ((State & MODE_INSERT) == 0 && *ml_get_buf(wp->w_buffer, wp->w_cursor.lnum, false) == NUL) {
+                   && *ml_get_buf(curwin->w_buffer, curwin->w_cursor.lnum, false) == NUL;
-    empty_line = true;
-  }
 
-  // Only draw the ruler when something changed.
-  validate_virtcol_win(wp);
-  if (redraw_cmdline
-      || always
-      || wp->w_cursor.lnum != wp->w_ru_cursor.lnum
-      || wp->w_cursor.col != wp->w_ru_cursor.col
-      || wp->w_virtcol != wp->w_ru_virtcol
-      || wp->w_cursor.coladd != wp->w_ru_cursor.coladd
-      || wp->w_topline != wp->w_ru_topline
-      || wp->w_buffer->b_ml.ml_line_count != wp->w_ru_line_count
-      || wp->w_topfill != wp->w_ru_topfill
-      || empty_line != wp->w_ru_empty) {
   int width;
   int row;
   int fillchar;
@@ -598,14 +584,6 @@ void win_redr_ruler(win_T *wp, bool always)
               this_ru_col + off + (int)strlen(buffer), off + width, fillchar,
               fillchar, attr);
   }
-
-    wp->w_ru_cursor = wp->w_cursor;
-    wp->w_ru_virtcol = wp->w_virtcol;
-    wp->w_ru_empty = (char)empty_line;
-    wp->w_ru_topline = wp->w_topline;
-    wp->w_ru_line_count = wp->w_buffer->b_ml.ml_line_count;
-    wp->w_ru_topfill = wp->w_topfill;
-  }
 }
 
 /// Get the character to use in a status line.  Get its attributes in "*attr".