patch 9.2.0035: syntax highlighting lost in popup with opacity

Problem:  syntax highlighting lost in popup with opacity lower than 100
          (after v9.2.0017)
Solution: Before blending, combine the popup's window color attribute
          with the character's own attribute using hl_combine_attr()
          (Yasuhiro Matsumoto).

related: #19272
closes:  #19478

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/screen.c

@@ -666,10 +666,15 @@ screen_line(
 	    opacity_blank = TRUE;
 	    // Keep the underlying character and blend its foreground color
 	    // from popup background color to original color.
+	    // Combine the popup window color with the character's own
+	    // attribute (e.g. match highlight) so that its background
+	    // color is preserved on blank cells.
+	    int char_attr = ScreenAttrs[off_from];
 	    int popup_attr = get_wcr_attr(screen_opacity_popup);
+	    int combined = hl_combine_attr(popup_attr, char_attr);
 	    int blend = screen_opacity_popup->w_popup_blend;
 	    ScreenAttrs[off_to] = hl_blend_attr(ScreenAttrs[off_to],
-						popup_attr, blend, TRUE);
+						combined, blend, TRUE);
 	    screen_char(off_to, row, col + coloff);
 	    // For wide background character, also update the second cell.
 	    if (bg_char_cells == 2)
@@ -818,11 +823,14 @@ skip_opacity:
 		    && (flags & SLF_POPUP)
 		    && screen_opacity_popup->w_popup_blend > 0)
 	    {
+		int char_attr = ScreenAttrs[off_from];
 		int popup_attr = get_wcr_attr(screen_opacity_popup);
 		int blend = screen_opacity_popup->w_popup_blend;
-		// Blend popup attr with default background (0)
-		// FALSE = keep popup foreground color, blend background only
-		ScreenAttrs[off_to] = hl_blend_attr(0, popup_attr, blend, FALSE);
+		// Combine popup window color with the character's own
+		// attribute (e.g. syntax highlighting) so that the
+		// character's foreground color is preserved.
+		int combined = hl_combine_attr(popup_attr, char_attr);
+		ScreenAttrs[off_to] = hl_blend_attr(0, combined, blend, FALSE);
 	    }
 #endif
 

src/testdir/dumps/Test_popupwin_opacity_hl_100.dump

@@ -0,0 +1,10 @@
+>1+0&#ffffff0| @73
+|2| @73
+|3| @7|f+0#ff404010#5fd7ff255|o@1| @1|b+0#0000000&|a|r| +0&#ffffff0@57
+|4| @7|b+0&#5fd7ff255|a|z| @4| +0&#ffffff0@57
+|5| @73
+|6| @73
+|7| @73
+|8| @73
+|9| @73
+@57|1|,|1| @10|T|o|p| 

src/testdir/dumps/Test_popupwin_opacity_hl_80.dump

@@ -0,0 +1,10 @@
+>1+0&#ffffff0| @73
+|2| @73
+|3| @7|f+0#ff404010#5fd7ff255|o@1| @1|b+0#0000000&|a|r| +0&#ffffff0@57
+|4| @7|b+0&#5fd7ff255|a|z| @4| +0&#ffffff0@57
+|5| @73
+|6| @73
+|7| @73
+|8| @73
+|9| @73
+|:| @55|1|,|1| @10|T|o|p| 

src/testdir/test_popupwin.vim

@@ -4653,6 +4653,40 @@ func Test_popupwin_bottom_position_without_decoration()
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_popup_opacity_highlight()
+  CheckScreendump
+
+  " Verify that match highlighting is preserved when opacity < 100,
+  " both for non-blank characters and trailing blank cells.
+  let lines =<< trim END
+	call setline(1, range(1, 100))
+	hi PopupColor ctermbg=lightblue
+	hi MyHl ctermfg=red
+
+	" 'foo  ' includes trailing spaces to test the opacity blank path.
+	let winid = popup_create(['foo  bar', 'baz'],
+	    \ #{line: 3, col: 10, highlight: 'PopupColor', opacity: 100})
+	call win_execute(winid, "call matchadd('MyHl', 'foo  ')")
+  END
+  call writefile(lines, 'XtestPopupOpacity', 'D')
+  let buf = RunVimInTerminal('-S XtestPopupOpacity', #{rows: 10})
+  call VerifyScreenDump(buf, 'Test_popupwin_opacity_hl_100', {})
+
+  " opacity=80: highlighted text should still be visible
+  call term_sendkeys(buf, ":call popup_clear()\<CR>")
+  call TermWait(buf)
+  call term_sendkeys(buf, ":let winid = popup_create(['foo  bar', 'baz'],"
+	\ .. " #{line: 3, col: 10, highlight: 'PopupColor', opacity: 80})\<CR>")
+  call TermWait(buf)
+  call term_sendkeys(buf, ":call win_execute(winid,"
+	\ .. " \"call matchadd('MyHl', 'foo  ')\")\<CR>")
+  call TermWait(buf)
+  call term_sendkeys(buf, ":\<CR>")
+  call VerifyScreenDump(buf, 'Test_popupwin_opacity_hl_80', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_popup_getwininfo_tabnr()
   tab split
   let winid1 = popup_create('sup', #{tabpage: 1})

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    35,
 /**/
     34,
 /**/