patch 9.1.1777: Mouse click to open fold doesn't work with utf-8 "foldclose" chars

Problem:  Mouse click to open fold doesn't work with utf-8 "foldclose"
          chars (Balki)
Solution: Use ScreenLinesUC[off] if it is set (zeertzjq).

fixes: #18344
closes: #18349

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/mouse.c

@@ -2109,7 +2109,8 @@ retnomove:
 #ifdef FEAT_FOLDING
 	// Remember the character under the mouse, it might be a '-' or '+' in
 	// the fold column.
-	mouse_char = ScreenLines[off];
+	mouse_char = enc_utf8 && ScreenLinesUC[off] != 0
+				       ? ScreenLinesUC[off] : ScreenLines[off];
 #endif
     }
 

src/testdir/test_termcodes.vim

@@ -1068,7 +1068,7 @@ func Test_mouse_alt_leftclick()
   bw!
 endfunc
 
-func Test_xterm_mouse_click_in_fold_columns()
+func Run_test_xterm_mouse_click_in_fold_columns()
   new
   let save_mouse = &mouse
   let save_term = &term
@@ -1120,6 +1120,15 @@ func Test_xterm_mouse_click_in_fold_columns()
   bwipe!
 endfunc
 
+func Test_xterm_mouse_click_in_fold_columns()
+  call Run_test_xterm_mouse_click_in_fold_columns()
+  set fillchars+=foldclose:▶
+  call Run_test_xterm_mouse_click_in_fold_columns()
+  set fillchars-=foldclose:▶ fillchars+=foldclose:!
+  call Run_test_xterm_mouse_click_in_fold_columns()
+  set fillchars&
+endfunc
+
 " Left or right click in Ex command line sets position of the cursor.
 func Test_term_mouse_click_in_cmdline_to_set_pos()
   let save_mouse = &mouse

src/version.c

@@ -724,6 +724,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1777,
 /**/
     1776,
 /**/