render: force underlines to be visible

In some cases, the underline position (typically provided by the font,
mind you), end up below the cell, making it visible.

Note that below the cell here means below the line. I.e. the font
provided underline position is below the font provided line height...

Oh well.

Doing this in foot rather than fcft, since other applications
typically don’t have to clip the rendered text.

Closes #503

CHANGELOG.md

@@ -75,8 +75,10 @@
 
 
 ### Deprecated
+
 * **bell** option in `foot.ini`; set actions in **[bell]** section instead.
 
+
 ### Removed
 
 * Buffer damage quirk for Plasma/KWin.
@@ -93,6 +95,9 @@
   (https://codeberg.org/dnkl/foot/issues/495).
 * Assertion when emitting a sixel occupying the entire scrollback
   history (https://codeberg.org/dnkl/foot/issues/494).
+* Font underlines being positioned below the cell (and thus being
+  invisible) for certain combinations of fonts and font sizes
+  (https://codeberg.org/dnkl/foot/issues/503).
 
 
 ### Security

render.c

@@ -317,11 +317,14 @@ draw_underline(const struct terminal *term, pixman_image_t *pix,
                const struct fcft_font *font,
                const pixman_color_t *color, int x, int y, int cols)
 {
+    /* Make sure the line isn't positioned below the cell */
+    int y_ofs = font_baseline(term) - font->underline.position;
+    y_ofs = min(y_ofs, term->cell_height - font->underline.thickness);
+
     pixman_image_fill_rectangles(
         PIXMAN_OP_SRC, pix, color,
         1, &(pixman_rectangle16_t){
-            x, y + font_baseline(term) - font->underline.position,
-            cols * term->cell_width, font->underline.thickness});
+            x, y + y_ofs, cols * term->cell_width, font->underline.thickness});
 }
 
 static void