patch 8.2.0554: the GUI doesn't set t_Co Commit: https://github.com/vim/vim/commit/acc770a10f65107131473ea2ab4ae5d429dfe172 Author: Bram Moolenaar <Bram@vim.org> Date: Sun Apr 12 15:11:06 2020 +0200 patch 8.2.0554: the GUI doesn't set t_Co Problem: The GUI doesn't set t_Co. Solution: In the GUI set t_Co to 256 * 256 * 256. (closes https://github.com/vim/vim/issues/5903)

--- a/src/gui.c
+++ b/src/gui.c
@@ -513,6 +513,9 @@ gui_init(void)
 	 */
 	set_option_value((char_u *)"paste", 0L, NULL, 0);
 
+	// Set t_Co to the number of colors: RGB.
+	set_color_count(256 * 256 * 256);
+
 	/*
 	 * Set up system-wide default menus.
 	 */