patch 9.2.0216: MS-Windows: Rendering artifacts with DirectX

Problem:  MS-Windows: Rendering artifacts with DirectX
          (Alexander Zhura)
Solution: Force redraw (Yasuhiro Matsumoto)

DirectWrite subpixel rendering (especially with CFF/OTF fonts) can
extend glyph pixels beyond cell boundaries.  Vim already handles the
forward direction (redraw the next character when the current one
changes) for MS-Windows antialiasing, but the backward direction was
missing.

Add gui.directx_enabled flag accessible from screen.c and extend the
existing spill-over handling to:
- Redraw the current character when the previous one changed (backward)
- Force redraw of the next character in screen_puts_len() and
  screen_fill() paths

fixes:  #19586
closes: #19761

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

src/gui.h

@@ -260,6 +260,9 @@ typedef struct Gui
     int		left_sbar_x;	    // Calculated x coord for left scrollbar
     int		right_sbar_x;	    // Calculated x coord for right scrollbar
     int         force_redraw;       // Force a redraw even e.g. not resized
+#ifdef FEAT_DIRECTX
+    int		directx_enabled;    // DirectX (DirectWrite) rendering active
+#endif
 
 #ifdef FEAT_MENU
 # ifndef FEAT_GUI_GTK

src/gui_w32.c

@@ -242,6 +242,7 @@ gui_mch_set_rendering_options(char_u *s)
 	}
     }
     s_directx_enabled = dx_enable;
+    gui.directx_enabled = IS_ENABLE_DIRECTX();
 
     return OK;
 # else

src/screen.c

@@ -920,6 +920,14 @@ skip_opacity:
 	    // redraw that one if this one changed, no matter attributes.
 	    if (gui.in_use && changed_this)
 		redraw_next = TRUE;
+# ifdef FEAT_DIRECTX
+	    // DirectWrite subpixel rendering (especially with CFF/OTF
+	    // fonts) can extend pixels beyond cell boundaries to the
+	    // left.  Redraw the current character if the previous one
+	    // changed.
+	    if (gui.directx_enabled && changed_this)
+		redraw_this = TRUE;
+# endif
 #endif
 
 	    ScreenAttrs[off_to] = ScreenAttrs[off_from];
@@ -1731,6 +1739,12 @@ screen_puts_len(
 		if (n & HL_BOLD)
 		    force_redraw_next = TRUE;
 	    }
+#endif
+#ifdef FEAT_DIRECTX
+	    // DirectWrite subpixel rendering can extend pixels beyond
+	    // cell boundaries.  Redraw the next character too.
+	    if (gui.directx_enabled && need_redraw)
+		force_redraw_next = TRUE;
 #endif
 	    // When at the end of the text and overwriting a two-cell
 	    // character with a one-cell character, need to clear the next
@@ -2663,6 +2677,12 @@ skip_opacity_fill:
 			force_next = FALSE;
 		}
 #endif // FEAT_GUI || defined(UNIX)
+#ifdef FEAT_DIRECTX
+		// DirectWrite subpixel rendering can extend pixels
+		// beyond cell boundaries.  Redraw the next character.
+		if (gui.directx_enabled)
+		    force_next = TRUE;
+#endif
 		ScreenLines[off] = c;
 		if (enc_utf8)
 		{

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    216,
 /**/
     215,
 /**/