foldcolumn: support "auto" (#13571)

"set foldcolumn=auto" is documented but not supported.
Support it by making it behave as "auto:1", similar to "signcolumn".

Close https://github.com/neovim/neovim/pull/13561

runtime/doc/options.txt

@@ -2438,7 +2438,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 'foldcolumn' 'fdc'	string (default "0")
 			local to window
 	When and how to draw the foldcolumn. Valid values are:
-	    "auto":       resize to the maximum amount of folds to display.
+	    "auto":       resize to the minimum amount of folds to display.
 	    "auto:[1-9]": resize to accommodate multiple folds up to the
 			  selected level
             0:            to disable foldcolumn

src/nvim/option.c

@@ -320,7 +320,7 @@ static char *(p_scl_values[]) =       { "yes", "no", "auto", "auto:1", "auto:2",
   "auto:3", "auto:4", "auto:5", "auto:6", "auto:7", "auto:8", "auto:9",
   "yes:1", "yes:2", "yes:3", "yes:4", "yes:5", "yes:6", "yes:7", "yes:8",
   "yes:9", "number", NULL };
-static char *(p_fdc_values[]) =       { "auto:1", "auto:2",
+static char *(p_fdc_values[]) =       { "auto", "auto:1", "auto:2",
   "auto:3", "auto:4", "auto:5", "auto:6", "auto:7", "auto:8", "auto:9",
   "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", NULL };
 

src/nvim/window.c

@@ -641,7 +641,7 @@ void win_set_minimal_style(win_T *wp)
     wp->w_p_scl = (char_u *)xstrdup("auto");
   }
 
-  // foldcolumn: use 'auto'
+  // foldcolumn: use '0'
   if (wp->w_p_fdc[0] != '0') {
     xfree(wp->w_p_fdc);
     wp->w_p_fdc = (char_u *)xstrdup("0");
@@ -700,9 +700,10 @@ int win_fdccol_count(win_T *wp)
   const char *fdc = (const char *)wp->w_p_fdc;
 
   // auto:<NUM>
-  if (strncmp(fdc, "auto:", 5) == 0) {
+  if (strncmp(fdc, "auto", 4) == 0) {
+    const int fdccol = fdc[4] == ':' ? fdc[5] - '0' : 1;
     int needed_fdccols = getDeepestNesting(wp);
-    return MIN(fdc[5] - '0', needed_fdccols);
+    return MIN(fdccol, needed_fdccols);
   } else {
     return fdc[0] - '0';
   }

test/functional/ui/fold_spec.lua

@@ -886,6 +886,41 @@ describe("folded lines", function()
                                                        |
         ]])
       end
+      command("set foldcolumn=auto")
+      if multigrid then
+        screen:expect{grid=[[
+        ## grid 1
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [3:---------------------------------------------]|
+        ## grid 2
+          {7:+}{5:^+--  2 lines: line 1························}|
+          {7: }line 3                                      |
+          {7: }line 4                                      |
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+        ## grid 3
+                                                       |
+        ]], unchanged=true}
+      else
+        screen:expect{grid=[[
+          {7:+}{5:^+--  2 lines: line 1························}|
+          {7: }line 3                                      |
+          {7: }line 4                                      |
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+                                                       |
+        ]], unchanged=true}
+      end
       -- fdc should not change with a new fold as the maximum is 1
       feed("zf3j")
 
@@ -924,6 +959,41 @@ describe("folded lines", function()
         ]])
       end
 
+      command("set foldcolumn=auto:1")
+      if multigrid then screen:expect{grid=[[
+        ## grid 1
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [2:---------------------------------------------]|
+          [3:---------------------------------------------]|
+        ## grid 2
+          {7:+}{5:^+--  4 lines: line 1························}|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+        ## grid 3
+                                                       |
+        ]], unchanged=true}
+      else
+        screen:expect{grid=[[
+          {7:+}{5:^+--  4 lines: line 1························}|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+          {1:~                                            }|
+                                                       |
+        ]], unchanged=true}
+      end
+
       -- relax the maximum fdc thus fdc should expand to
       -- accomodate the current number of folds
       command("set foldcolumn=auto:4")