Crude patch to limit width of a fold line in Vim.

vim-limited-fold-width.diff

diff --git a/src/screen.c b/src/screen.c
index d80ad6f..fbd2c41 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -2366,6 +2366,10 @@ compute_foldcolumn(wp, col)
 
     if (fdc > wwidth - (col + wmw))
 	fdc = wwidth - (col + wmw);
+
+    if (fdc > 80)
+        fdc = 80;
+
     return fdc;
 }
 
@@ -2455,9 +2459,22 @@ fold_line(wp, fold_count, foldinfo, lnum, row)
 				 ScreenAttrs[off + (p) + ri] = v
 #endif
 
+    const int fold_len = number_width(wp) + comp_textwidth(FALSE) + 1;
+    int max_col;
+
     /* Set all attributes of the 'number' or 'relativenumber' column and the
      * text */
-    RL_MEMSET(col, hl_attr(HLF_FL), W_WIDTH(wp) - col);
+    if (fold_len >= W_WIDTH(wp) - col)
+    {
+        RL_MEMSET(col, hl_attr(HLF_FL), W_WIDTH(wp) - col);
+        max_col = W_WIDTH(wp);
+    }
+    else
+    {
+        RL_MEMSET(col, hl_attr(HLF_FL), fold_len);
+        RL_MEMSET(col + fold_len, hl_attr(HLF_AT), W_WIDTH(wp) - col - fold_len);
+        max_col = fold_len;
+    }
 
 #ifdef FEAT_SIGNS
     /* If signs are being displayed, add two spaces. */
@@ -2665,7 +2682,7 @@ fold_line(wp, fold_count, foldinfo, lnum, row)
     if (wp->w_p_rl)
 	col -= txtcol;
 #endif
-    while (col < W_WIDTH(wp)
+    while (col < max_col
 #ifdef FEAT_RIGHTLEFT
 		    - (wp->w_p_rl ? txtcol : 0)
 #endif

Demonstration (left part is with the patch applied, right part is without it):