patch 8.2.4906: MS-Windows: cannot use transparent background

Problem:    MS-Windows: cannot use transparent background.
Solution:   Make transparent background work with 'termguicolors' and NONE
            background color. (Yasuhiro Matsumoto, closes #10310, closes #7162)

runtime/doc/options.txt

@@ -8079,6 +8079,13 @@ A jump table for the options with a short description can be found at |Q_op|.
 <	This requires Vim to be built with the |+vtp| feature.
 
 	Note that the "cterm" attributes are still used, not the "gui" ones.
+
+	When using Vim with Windows Terminal, the background of Windows
+	Terminal is normally filled with the Vim background color.  Setting
+	'termguicolors' and the guibg of the Normal highlight group to NONE
+	will make the background transparent: >
+		:hi Normal guibg=NONE
+<
 	NOTE: This option is reset when 'compatible' is set.
 
 						*'termwinkey'* *'twk'*

src/os_win32.c

@@ -211,7 +211,6 @@ static int g_color_index_bg = 0;
 static int g_color_index_fg = 7;
 
 # ifdef FEAT_TERMGUICOLORS
-static int default_console_color_bg = 0x000000; // black
 static int default_console_color_fg = 0xc0c0c0; // white
 # endif
 
@@ -7846,7 +7845,7 @@ vtp_init(void)
     HMODULE hKerneldll;
     DYN_CONSOLE_SCREEN_BUFFER_INFOEX csbi;
 # ifdef FEAT_TERMGUICOLORS
-    COLORREF fg, bg;
+    COLORREF fg;
 # endif
 
     // Use functions supported from Vista
@@ -7873,11 +7872,8 @@ vtp_init(void)
     store_console_fg_rgb = save_console_fg_rgb;
 
 # ifdef FEAT_TERMGUICOLORS
-    bg = (COLORREF)csbi.ColorTable[g_color_index_bg];
     fg = (COLORREF)csbi.ColorTable[g_color_index_fg];
-    bg = (GetRValue(bg) << 16) | (GetGValue(bg) << 8) | GetBValue(bg);
     fg = (GetRValue(fg) << 16) | (GetGValue(fg) << 8) | GetBValue(fg);
-    default_console_color_bg = bg;
     default_console_color_fg = fg;
 # endif
 
@@ -8158,10 +8154,11 @@ get_default_console_color(
 	ctermbg = -1;
 	if (id > 0)
 	    syn_id2cterm_bg(id, &ctermfg, &ctermbg);
-	guibg = ctermbg != -1 ? ctermtoxterm(ctermbg)
-						    : default_console_color_bg;
-	cterm_normal_bg_gui_color = guibg;
-	ctermbg = ctermbg < 0 ? 0 : ctermbg;
+	cterm_normal_bg_gui_color = guibg =
+			    ctermbg != -1 ? ctermtoxterm(ctermbg) : INVALCOLOR;
+
+	if (ctermbg < 0)
+	    ctermbg = 0;
     }
 
     *cterm_fg = ctermfg;

src/term.c

@@ -3084,7 +3084,8 @@ term_fg_rgb_color(guicolor_T rgb)
     void
 term_bg_rgb_color(guicolor_T rgb)
 {
-    term_rgb_color(T_8B, rgb);
+    if (rgb != INVALCOLOR)
+	term_rgb_color(T_8B, rgb);
 }
 
     void

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4906,
 /**/
     4905,
 /**/