[PATCH] Skip ZERO_WIDTH_CHAR to avoid rendering artifacts with background colors

[Ross Schlaikjer]

Hello,

I was seeing artifacting over text input when using an NPM TUI that was
drawing zero-width spaces (U+200B) over the input as some kind of cursor
rendering trick. The outline of the placeholder glyph would display
repeatedly as a ghost outline, not disappearing until I resized the window
to force a redraw.

It seems like the rxvt_font_default::draw() method handles zero-width
characters by just skipping them, so I applied this to the _xft and _x11
versions as well, which appears to have fixed the issue (my font is
xft:Unifont:pixelsize=24).

Screenshots of the terminal before/after the patch:
Before: https://www.xvjpf.org/rxvt_zwsp_before.png
After: https://www.xvjpf.org/rxvt_zwsp_after.png

Hopefully this patch (appended) is generally useful.

Best,
- Ross

--

diff --git a/src/rxvtfont.C b/src/rxvtfont.C
index 2a36e96..8e4ae8b 100644
--- a/src/rxvtfont.C
+++ b/src/rxvtfont.C
@@ -1064,6 +1064,10 @@ rxvt_font_x11::draw (rxvt_drawable &d, int x, int y,

   int base = ascent; // sorry, incorrect: term->fbase;

+  // skip zero-width characters to avoid rendering artifacts with
background colors
+  while (len && *text == ZERO_WIDTH_CHAR)
+    text++, len--;
+
   XGCValues v;
   v.foreground = term->pix_colors[fg];
   v.font = f->fid;
@@ -1452,6 +1456,10 @@ rxvt_font_xft::draw (rxvt_drawable &d, int x, int y,
       while (len && *text == NOCHAR)
         text++, len--, cwidth += term->fwidth;

+      if (chr == ZERO_WIDTH_CHAR)
+        // skip zero-width characters to avoid rendering artifacts with
background colors
+        continue;
+
       if (chr != ' ') // skip spaces
         {
           // handle non-bmp chars when text_t is 16 bit
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