fix(statusline): broken statusline on error #38000

Problem: after #33036, an error from evaluating 'statusline' clears it and
doesn't draw the statusline. (causing glitchy redraws)

Solution: use the default value instead. If 'stl' is somehow ever empty, still
call redraw_custom_statusline to at least draw an empty statusline.

Ideally our default 'stl' shouldn't itself error too! :-)
Also adjust some prior screen:expect()s to avoid immediate success warnings.

runtime/doc/options.txt

@@ -6286,10 +6286,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 	current window and buffer, while %{} items are evaluated in the
 	context of the window that the statusline belongs to.
 
-	When there is error while evaluating the option then it will be made
+	When there is an error while evaluating the option it will be reset to
-	empty to avoid further errors.  Otherwise screen updating would loop.
+	its default value to avoid further errors.  Otherwise screen updating
-	When the result contains unprintable characters the result is
+	would loop.  When the result contains unprintable characters the
-	unpredictable.
+	result is unpredictable.
 
 	Note that the only effect of 'ruler' when this option is set (and
 	'laststatus' is 2 or 3) is controlling the output of |CTRL-G|.

runtime/lua/vim/_meta/options.lua

@@ -6728,10 +6728,10 @@ vim.wo.stc = vim.wo.statuscolumn
 --- current window and buffer, while %{} items are evaluated in the
 --- context of the window that the statusline belongs to.
 ---
---- When there is error while evaluating the option then it will be made
+--- When there is an error while evaluating the option it will be reset to
---- empty to avoid further errors.  Otherwise screen updating would loop.
+--- its default value to avoid further errors.  Otherwise screen updating
---- When the result contains unprintable characters the result is
+--- would loop.  When the result contains unprintable characters the
---- unpredictable.
+--- result is unpredictable.
 ---
 --- Note that the only effect of 'ruler' when this option is set (and
 --- 'laststatus' is 2 or 3) is controlling the output of `CTRL-G`.

src/nvim/options.lua

@@ -8794,10 +8794,10 @@ local options = {
         current window and buffer, while %{} items are evaluated in the
         context of the window that the statusline belongs to.
 
-        When there is error while evaluating the option then it will be made
+        When there is an error while evaluating the option it will be reset to
-        empty to avoid further errors.  Otherwise screen updating would loop.
+        its default value to avoid further errors.  Otherwise screen updating
-        When the result contains unprintable characters the result is
+        would loop.  When the result contains unprintable characters the
-        unpredictable.
+        result is unpredictable.
 
         Note that the only effect of 'ruler' when this option is set (and
         'laststatus' is 2 or 3) is controlling the output of |CTRL-G|.

src/nvim/statusline.c

@@ -85,8 +85,7 @@ void win_redr_status(win_T *wp)
     // Don't redraw right now, do it later. Don't update status line when
     // popup menu is visible and may be drawn over it
     wp->w_redr_status = true;
-  } else if (*wp->w_p_stl != NUL
+  } else if (*wp->w_p_stl != NUL || !wp->w_floating || (is_stl_global && wp == curwin)) {
-             || (*p_stl != NUL && (!wp->w_floating || (is_stl_global && wp == curwin)))) {
     // redraw custom status line
     redraw_custom_statusline(wp);
   }
@@ -2136,13 +2135,12 @@ stcsign:
   redraw_not_allowed = save_redraw_not_allowed;
 
   // Check for an error.  If there is one the display will be messed up and
-  // might loop redrawing.  Avoid that by making the corresponding option
+  // might loop redrawing.  Avoid that by setting the option to its default.
-  // empty.
   // TODO(Bram): find out why using called_emsg_before makes tests fail, does it
   // matter?
   // if (called_emsg > called_emsg_before)
   if (opt_idx != kOptInvalid && did_emsg > did_emsg_before) {
-    set_option_direct(opt_idx, STATIC_CSTR_AS_OPTVAL(""), opt_scope, SID_ERROR);
+    set_option_direct(opt_idx, get_option_default(opt_idx, opt_scope), opt_scope, SID_ERROR);
   }
 
   // A user function may reset KeyTyped, restore it.

test/functional/ui/statusline_spec.lua

@@ -949,6 +949,12 @@ describe('default statusline', function()
   it('setting statusline to empty string sets default statusline', function()
     exec_lua("vim.o.statusline = 'asdf'")
     eq('asdf', eval('&statusline'))
+    screen:expect([[
+      ^                                                            |
+      {1:~                                                           }|*13
+      {3:asdf                                                        }|
+                                                                  |
+    ]])
 
     local default_statusline = table.concat({
       '%<',
@@ -962,15 +968,37 @@ describe('default statusline', function()
     })
 
     exec_lua("vim.o.statusline = ''")
-
     eq(default_statusline, eval('&statusline'))
-
     screen:expect([[
       ^                                                            |
       {1:~                                                           }|*13
       {3:[No Name]                                 0,0-1          All}|
                                                                   |
     ]])
+
+    -- Reset to default (via the correct scope) if there's an error.
+    command('setglobal statusline=%{a%}')
+    eq(default_statusline, eval('&statusline'))
+    eq(default_statusline, eval('&g:statusline'))
+    eq('', eval('&l:statusline'))
+    command('redrawstatus') -- like Vim, statusline isn't immediately redrawn after an error
+    screen:expect([[
+      ^                                                            |
+      {1:~                                                           }|*13
+      {3:[No Name]                                 0,0-1          All}|
+      {9:E121: Undefined variable: a}                                 |
+    ]])
+    command('setlocal statusline=%{b%}')
+    eq(default_statusline, eval('&statusline'))
+    eq(default_statusline, eval('&g:statusline'))
+    eq('', eval('&l:statusline'))
+    command('redrawstatus') -- like Vim, statusline isn't immediately redrawn after an error
+    screen:expect([[
+      ^                                                            |
+      {1:~                                                           }|*13
+      {3:[No Name]                                 0,0-1          All}|
+      {9:E121: Undefined variable: b}                                 |
+    ]])
   end)
 
   it('shows busy status when buffer is set to be busy', function()
@@ -1064,7 +1092,7 @@ describe("'statusline' in floatwin", function()
 
     -- no statusline is displayed because the statusline option was cleared
     api.nvim_win_set_config(win, cfg)
-    screen:expect(without_stl)
+    screen:expect_unchanged()
 
     -- displayed after the option is reset
     command(set_stl)
@@ -1090,6 +1118,19 @@ describe("'statusline' in floatwin", function()
     ]])
     command('tabfirst')
     api.nvim_win_set_config(win2, { style = 'minimal' })
+    screen:expect([[
+      {5: }{101:2}{5:+ [No Name] }{24: }{102:2}{24:+ [No Name] }{2: }{24:X}|
+       ┌──────────┐                 |
+      {1:~}│{4:^1         }│{1:                 }|
+      {1:~}│{4:2         }│{1:                 }|
+      {1:~}│{4:3         }│{1:                 }|
+      {1:~}│{4:4         }│{1:                 }|
+      {1:~}│{3:<Name] [+]}│{1:                 }|
+      {1:~}└──────────┘{1:                 }|
+      {1:~                             }|*10
+      {2:[No Name]                     }|
+                                    |
+    ]])
     command('tabnext')
     screen:expect([[
       {24: }{102:2}{24:+ [No Name] }{5: }{101:2}{5:+ [No Name] }{2: }{24:X}|

test/old/testdir/test_statusline.vim

@@ -64,7 +64,9 @@ func Test_statusline_will_be_disabled_with_error()
   catch
   endtry
   call assert_true(s:func_in_statusline_called)
-  call assert_equal('', &statusline)
+  " Nvim: resets to default value instead.
+  " call assert_equal('', &statusline)
+  call assert_equal(nvim_get_option_info2('statusline', {}).default, &statusline)
   set statusline=
 endfunc