sixel: limit image size to the one specified in the raster attrs

If a sixel has set an explicit width/height, in its raster attributes,
do *not* allow the image to extend beyond this.

Before this patch, we dynamically resized images if the sixel encoder
emitted sixel data beyond the previously set width/height.

Now, we ignore writes outside the configured size.

CHANGELOG.md

@@ -107,6 +107,9 @@
   determining whether a key combination produces text or not.
 * CSI-t queries now report pixel values **unscaled**, instead of
   **scaled** ([#1643][1643]).
+* Sixel "level 2" images (raster attributes set) are no longer
+  extended beyond the width/height specified in the raster attributes
+  ([#chafa-192][chafa-192]).
 
 [1526]: https://codeberg.org/dnkl/foot/issues/1526
 [1528]: https://codeberg.org/dnkl/foot/issues/1528
@@ -114,6 +117,7 @@
 [kitty-6913]: https://github.com/kovidgoyal/kitty/issues/6913
 [1584]: https://codeberg.org/dnkl/foot/issues/1584
 [1643]: https://codeberg.org/dnkl/foot/issues/1643
+[chafa-192]: https://github.com/hpjansson/chafa/issues/192
 
 
 ### Deprecated

sixel.c

@@ -82,6 +82,7 @@ sixel_init(struct terminal *term, int p1, int p2, int p3)
     term->sixel.color_idx = 0;
     term->sixel.pan = pan;
     term->sixel.pad = pad;
+    term->sixel.size_locked = false;
     term->sixel.param = 0;
     term->sixel.param_idx = 0;
     memset(term->sixel.params, 0, sizeof(term->sixel.params));
@@ -90,6 +91,7 @@ sixel_init(struct terminal *term, int p1, int p2, int p3)
     term->sixel.image.p = NULL;
     term->sixel.image.width = 0;
     term->sixel.image.height = 0;
+    term->sixel.image.alloc_height = 0;
 
     if (term->sixel.use_private_palette) {
         xassert(term->sixel.private_palette == NULL);
@@ -1299,6 +1301,7 @@ sixel_unhook(struct terminal *term)
     term->sixel.image.p = NULL;
     term->sixel.image.width = 0;
     term->sixel.image.height = 0;
+    term->sixel.image.alloc_height = 0;
     term->sixel.pos = (struct coord){0, 0};
 
     free(term->sixel.private_palette);
@@ -1314,6 +1317,9 @@ sixel_unhook(struct terminal *term)
 static void
 resize_horizontally(struct terminal *term, int new_width)
 {
+    if (likely(term->sixel.size_locked))
+        return;
+
     if (unlikely(new_width > term->sixel.max_width)) {
         LOG_WARN("maximum image dimensions exceeded, truncating");
         new_width = term->sixel.max_width;
@@ -1375,6 +1381,9 @@ resize_vertically(struct terminal *term, int new_height)
             term->sixel.image.width, term->sixel.image.height,
             term->sixel.image.width, new_height);
 
+    if (likely(term->sixel.size_locked))
+        return false;
+
     if (unlikely(new_height > term->sixel.max_height)) {
         LOG_WARN("maximum image dimensions reached");
         return false;
@@ -1493,6 +1502,7 @@ resize(struct terminal *term, int new_width, int new_height)
     term->sixel.image.width = new_width;
     term->sixel.image.height = new_height;
     term->sixel.image.p = &term->sixel.image.data[term->sixel.pos.row * new_width + term->sixel.pos.col];
+    term->sixel.image.alloc_height = alloc_new_height;
 
     return true;
 }
@@ -1663,7 +1673,7 @@ decsixel_generic(struct terminal *term, uint8_t c)
         term->sixel.pos.col = 0;
         term->sixel.image.p = &term->sixel.image.data[term->sixel.pos.row * term->sixel.image.width];
 
-        if (term->sixel.pos.row >= term->sixel.image.height) {
+        if (unlikely(term->sixel.pos.row >= term->sixel.image.alloc_height)) {
             if (!resize_vertically(term, term->sixel.pos.row + 6 * term->sixel.pan))
                 term->sixel.pos.col = term->sixel.max_width + 1 * term->sixel.pad;
         }
@@ -1737,6 +1747,7 @@ decgra(struct terminal *term, uint8_t c)
             ph <= term->sixel.max_height && pv <= term->sixel.max_width)
         {
             resize(term, ph, pv);
+            term->sixel.size_locked = true;
         }
 
         term->sixel.state = SIXEL_DECSIXEL;

terminal.h

@@ -677,6 +677,7 @@ struct terminal {
             uint32_t *p;     /* Pointer into data, for current position */
             int width;       /* Image width, in pixels */
             int height;      /* Image height, in pixels */
+            int alloc_height;  /* Image height, in pixels, aligned to 6-pixel boundary */
         } image;
 
         /*
@@ -687,6 +688,7 @@ struct terminal {
          */
         int pan;
         int pad;
+        bool size_locked;
 
         bool scrolling:1;                 /* Private mode 80 */
         bool use_private_palette:1;       /* Private mode 1070 */