config: add optional ‘center’ argument to ‘pad’

When set, the grid contents is centered in the window. I.e. the
left/right and top/bottom margins are equal (+- 1px).

This causes the content to “jump” while doing an interactive resize,
but may still be preferred in e.g. a tiling WM.

Closes #273

CHANGELOG.md

@@ -23,6 +23,13 @@
 
 ## Unreleased
 ### Added
+
+* The `pad` option now accepts an optional third argument, `center`
+  (e.g. `pad=5x5 center`), causing the grid to be centered in the
+  window, with equal amount of padding of the left/right and
+  top/bottom side (https://codeberg.org/dnkl/foot/issues/273).
+
+
 ### Changed
 
 * The fcft and tllist library subprojects are now handled via Meson

config.c

@@ -466,9 +466,15 @@ parse_section_main(const char *key, const char *value, struct config *conf,
 
     else if (strcmp(key, "pad") == 0) {
         unsigned x, y;
-        if (sscanf(value, "%ux%u", &x, &y) != 2) {
+        char mode[16] = {0};
+
+        int ret = sscanf(value, "%ux%u %15s", &x, &y, mode);
+        bool center = strcasecmp(mode, "center") == 0;
+        bool invalid_mode = !center && mode[0] != '\0';
+
+        if ((ret != 2 && ret != 3) || invalid_mode) {
             LOG_AND_NOTIFY_ERR(
-                "%s:%d: [default]: pad: expected PAD_XxPAD_Y, "
+                "%s:%d: [default]: pad: expected PAD_XxPAD_Y [center], "
                 "where both are positive integers, got '%s'",
                 path, lineno, value);
             return false;
@@ -476,6 +482,7 @@ parse_section_main(const char *key, const char *value, struct config *conf,
 
         conf->pad_x = x;
         conf->pad_y = y;
+        conf->center = center;
     }
 
     else if (strcmp(key, "bold-text-in-bright") == 0)

config.h

@@ -70,6 +70,7 @@ struct config {
 
     unsigned pad_x;
     unsigned pad_y;
+    bool center;
 
     bool bold_in_bright;
     enum {

doc/foot.ini.5.scd

@@ -73,7 +73,18 @@ in this order:
 
 *pad*
 	Padding between border and glyphs, in pixels (subject to output
-	scaling), on the form _XxY_. Default: _2x2_.
+	scaling), on the form _XxY_.
+	
+	This will add _at least_ X pixels on both the left and right
+	sides, and Y pixels on the top and bottom sides. The grid content
+	will be anchored in the top left corner. I.e. if the window
+	manager forces an odd window size on foot, the additional pixels
+	will be added to the right and bottom sides.
+	
+	To instead center the grid content, append *center* (e.g. *pad=5x5
+	center*).
+	
+	Default: _2x2_.
 
 *initial-window-size-pixels*
 	Initial window width and height in _pixels_ (subject to output

foot.ini

@@ -8,7 +8,7 @@
 # initial-window-size-pixels=700x500  # Or,
 # initial-window-size-chars=<COLSxROWS>
 # initial-window-mode=windowed
-# pad=2x2
+# pad=2x2                             # optionally append 'center'
 # shell=$SHELL (if set, otherwise user's default shell from /etc/passwd)
 # term=foot
 # login-shell=no

render.c

@@ -2638,10 +2638,20 @@ maybe_resize(struct terminal *term, int width, int height, bool force)
     assert(new_rows >= 1);
 
     /* Margins */
-    term->margins.left = pad_x;
-    term->margins.top = pad_y;
-    term->margins.right = term->width - new_cols * term->cell_width - term->margins.left;
-    term->margins.bottom = term->height - new_rows * term->cell_height - term->margins.top;
+    const int grid_width = new_cols * term->cell_width;
+    const int grid_height = new_rows * term->cell_height;
+    const int total_x_pad = term->width - grid_width;
+    const int total_y_pad = term->height - grid_height;
+
+    if (term->conf->center) {
+        term->margins.left = total_x_pad / 2;
+        term->margins.top = total_y_pad / 2;
+    } else {
+        term->margins.left = pad_x;
+        term->margins.top = pad_y;
+    }
+    term->margins.right = total_x_pad - term->margins.left;
+    term->margins.bottom = total_y_pad - term->margins.top;
 
     assert(term->margins.left >= pad_x);
     assert(term->margins.right >= pad_x);