fix(conceal): check for concealed lines on invalid row (#36084)

Problem:  May check for concealed lines on invalid line numbers.
Solution: Move checks after line number validation.

src/nvim/decoration.c

@@ -865,6 +865,7 @@ static const uint32_t conceal_filter[kMTMetaCount] = {[kMTMetaConcealLines] = kM
 /// @return whether "row" is concealed
 bool decor_conceal_line(win_T *wp, int row, bool check_cursor)
 {
+  assert(row >= 0);
   if (wp->w_p_cole < 2
       || (!check_cursor && wp == curwin && row + 1 == wp->w_cursor.lnum
           && !conceal_cursor_line(wp))) {

src/nvim/edit.c

@@ -2449,10 +2449,10 @@ void cursor_up_inner(win_T *wp, linenr_T n, bool skip_conceal)
     while (n--) {
       // move up one line
       lnum--;
-      n += skip_conceal && decor_conceal_line(wp, lnum - 1, true);
       if (lnum <= 1) {
         break;
       }
+      n += skip_conceal && decor_conceal_line(wp, lnum - 1, true);
       // If we entered a fold, move to the beginning, unless in
       // Insert mode or when 'foldopen' contains "all": it will open
       // in a moment.
@@ -2506,10 +2506,10 @@ void cursor_down_inner(win_T *wp, int n, bool skip_conceal)
       } else {
         lnum++;
       }
-      n += skip_conceal && decor_conceal_line(wp, lnum - 1, true);
       if (lnum >= line_count) {
         break;
       }
+      n += skip_conceal && decor_conceal_line(wp, lnum - 1, true);
     }
     lnum = MIN(lnum, line_count);
   } else {

src/nvim/move.c

@@ -331,10 +331,10 @@ void update_topline(win_T *wp)
         // scrolled).
         n = 0;
         for (linenr_T lnum = wp->w_cursor.lnum; lnum < wp->w_topline + *so_ptr; lnum++) {
-          n += !decor_conceal_line(wp, lnum, false);
           // stop at end of file or when we know we are far off
           assert(wp->w_buffer != 0);
-          if (lnum >= wp->w_buffer->b_ml.ml_line_count || n >= halfheight) {
+          if (lnum >= wp->w_buffer->b_ml.ml_line_count
+              || (n += !decor_conceal_line(wp, lnum, false)) >= halfheight) {
             break;
           }
           hasFolding(wp, lnum, NULL, &lnum);
@@ -404,23 +404,23 @@ void update_topline(win_T *wp)
         }
       }
       if (check_botline) {
-        int line_count = 0;
+        int n = 0;
         if (win_lines_concealed(wp)) {
           // Count the number of logical lines between the cursor and
           // botline - p_so (approximation of how much will be
           // scrolled).
           for (linenr_T lnum = wp->w_cursor.lnum; lnum >= wp->w_botline - *so_ptr; lnum--) {
-            line_count += !decor_conceal_line(wp, lnum - 1, false);
             // stop at end of file or when we know we are far off
-            if (lnum <= 0 || line_count > wp->w_view_height + 1) {
+            if (lnum <= 0
+                || (n += !decor_conceal_line(wp, lnum - 1, false)) > wp->w_view_height + 1) {
               break;
             }
             hasFolding(wp, lnum, &lnum, NULL);
           }
         } else {
-          line_count = wp->w_cursor.lnum - wp->w_botline + 1 + (int)(*so_ptr);
+          n = wp->w_cursor.lnum - wp->w_botline + 1 + (int)(*so_ptr);
         }
-        if (line_count <= wp->w_view_height + 1) {
+        if (n <= wp->w_view_height + 1) {
           scroll_cursor_bot(wp, scrolljump_value(wp), false);
         } else {
           scroll_cursor_halfway(wp, false, false);
@@ -513,7 +513,7 @@ void check_cursor_moved(win_T *wp)
                      |VALID_CHEIGHT|VALID_CROW|VALID_TOPLINE);
 
     // Concealed line visibility toggled.
-    if (wp->w_p_cole >= 2 && !conceal_cursor_line(wp)
+    if (wp->w_valid_cursor.lnum > 0 && wp->w_p_cole >= 2 && !conceal_cursor_line(wp)
         && (decor_conceal_line(wp, wp->w_cursor.lnum - 1, true)
             || decor_conceal_line(wp, wp->w_valid_cursor.lnum - 1, true))) {
       changed_window_setting(wp);

test/functional/ui/decorations_spec.lua

@@ -3524,6 +3524,17 @@ describe('extmark decorations', function()
     eq(5, n.fn.line('w0'))
   end)
 
+  it('conceal_lines not checking on invalid row #36057', function()
+    exec_lua(function()
+      vim.fn.setline(1, { 'foo', 'bar', 'baz' })
+      vim.api.nvim_command('set conceallevel=3 scrolloff=3')
+      vim.api.nvim_open_win(0, true, { width = 1, height = 1, relative = 'editor', row = 0, col = 0 })
+      vim.api.nvim_buf_set_extmark(0, ns, 1, 0, { conceal_lines = '' })
+      vim.api.nvim__redraw({ flush = true })
+    end)
+    n.assert_alive()
+  end)
+
   it('redraws the line from which a left gravity mark has moved #27369', function()
     fn.setline(1, { 'aaa', 'bbb', 'ccc', 'ddd' })
     api.nvim_buf_set_extmark(0, ns, 1, 0, { virt_text = { { 'foo' } }, right_gravity = false })