layout: a float sized but not positioned is centred, per axis

docs/layouts.md always said an absent coordinate takes the centred
default; the loader said 0, and a layout float with only w= h= landed
in the top-left corner. Now an axis the file does not name is
FLOAT_CENTRE, a sentinel no real coordinate can reach, and float_clamp
answers it with the middle -- every frame, at whatever size the
terminal is by then, until an explicit move or resize writes a real
number over it. The webdemo's welcome float is the customer: it must
sit centred on a terminal whose size the layout file cannot know.

The dump writes a still-centred axis by omission, because the number
it happened to land on would pin to this screen's size the one intent
that deliberately has none. Every mutating verb already writes back
from the drawn rect, so the sentinel survives nothing it should not.

1 files changed, 16 insertions, 7 deletions

diff --git a/src/app_session.c b/src/app_session.c
index 1e78d9e..716b5d1 100644
--- a/src/app_session.c
+++ b/src/app_session.c
@@ -123,12 +123,16 @@ static node_t *build_pane(app_t *a, const kdl_node_t *node, const char *cwd,
   if (kdl_prop_bool(node, "floating", false)) {
     leaf->floating = true;
     leaf->raised = ++a->raise_seq;
-    long fx = kdl_prop_int(node, "x", 0), fy = kdl_prop_int(node, "y", 0);
+    /* A coordinate the file does not name is centred (FLOAT_CENTRE), not
+     * zero: `w=` `h=` alone means "this big, wherever the middle is", and
+     * it stays the middle through every resize until something writes a
+     * real coordinate — which is what the docs promised all along. */
+    long fx = kdl_prop_int(node, "x", -1), fy = kdl_prop_int(node, "y", -1);
     long fw = kdl_prop_int(node, "w", 0), fh = kdl_prop_int(node, "h", 0);
     if (fw > 0 && fh > 0)
-      leaf->float_rect =
-          (rect_t){(uint16_t)(fx < 0 ? 0 : fx), (uint16_t)(fy < 0 ? 0 : fy),
-                   (uint16_t)fw, (uint16_t)fh};
+      leaf->float_rect = (rect_t){(uint16_t)(fx < 0 ? FLOAT_CENTRE : fx),
+                                  (uint16_t)(fy < 0 ? FLOAT_CENTRE : fy),
+                                  (uint16_t)fw, (uint16_t)fh};
   }
   /* `focus=true` restores which pane you were in. Recorded on the node and
    * resolved once the tab exists, because focus belongs to the tab. */
@@ -915,9 +919,14 @@ static void dump_node(node_t *n, strbuf_t *b, int depth, dumpctx_t *ctx) {
    * exactly is a session, not a shape. */
   if (n->floating) {
     sb_add(b, " floating=true");
-    if (n->float_rect.w)
-      sb_add(b, " x=%u y=%u w=%u h=%u", n->float_rect.x, n->float_rect.y,
-             n->float_rect.w, n->float_rect.h);
+    /* An axis still centred (FLOAT_CENTRE) is dumped by omission: writing
+     * the number it happened to land on would pin to this screen's size the
+     * one intent that deliberately has none. */
+    if (n->float_rect.w) {
+      if (n->float_rect.x != FLOAT_CENTRE) sb_add(b, " x=%u", n->float_rect.x);
+      if (n->float_rect.y != FLOAT_CENTRE) sb_add(b, " y=%u", n->float_rect.y);
+      sb_add(b, " w=%u h=%u", n->float_rect.w, n->float_rect.h);
+    }
   }
   if (n == ctx->tab->focus) sb_add(b, " focus=true");
   sb_add(b, "\n");