fixed new participants not detected in syncs

fixes #20

src/core/draw/sync.typ

@@ -4,52 +4,62 @@
 #let render(sync) = get-ctx(ctx => {
   set-ctx(c => {
     c.sync = (
+      ctx: ctx,
       bottoms: (),
       starts: (),
-      y: ctx.y
+      start-y: ctx.y,
+      align-y: ctx.y
     )
+    c.in-sync = true
     return c
   })
+})
 
-  // Compute heights
+#let in-sync-render(elmt) = {
+  set-ctx(c => {
+    c.y = c.sync.start-y
+    return c
+  })
+  draw.hide({
+    (elmt.draw)(elmt)
+  })
+  set-ctx(c => {
+    c.sync.starts.push(c.last-drawn.start-info.y)
+    c.sync.bottoms.push(c.y)
+    return c
+  })
+}
+
+#let render-end(sync) = get-ctx(ctx => {
   for e in sync.elmts {
     assert(is-elmt(e), message: "Sync element can only contain chronos elements, found " + repr(e))
     assert(
       e.type == "seq",
       message: "Sync element can only contain sequences, found '" + e.type + "'"
     )
-
-    set-ctx(c => {
-      c.y = ctx.y
-      return c
-    })
-    draw.hide({
-      (e.draw)(e)
-    })
-    set-ctx(c => {
-      c.sync.starts.push(c.last-drawn.start-info.y)
-      c.sync.bottoms.push(c.y)
-      return c
-    })
   }
 
   set-ctx(c => {
-    c.sync-y = calc.min(..c.sync.starts)
-    return c
+    let new-sync = c.sync
+    if new-sync.starts.len() != 0 {
+      new-sync.align-y = calc.min(..new-sync.starts)
+    }
+    new-sync.remove("ctx")
+    return c.sync.ctx + (sync: new-sync)
   })
 
-  // Draw aligned elements
   for (i, e) in sync.elmts.enumerate() {
     set-ctx(c => {
-      let dy = c.sync.starts.at(i) - ctx.y
-      c.y = c.sync-y - dy
+      let dy = c.sync.starts.at(i) - c.sync.start-y
+      c.y = c.sync.align-y - dy
       return c
     })
     (e.draw)(e)
   }
 
   set-ctx(c => {
-    c.y = calc.min(..c.sync.bottoms)
+    let heights = c.sync.starts.zip(c.sync.bottoms).map(((s, b)) => b - s)
+    c.y = c.sync.align-y + calc.min(..heights)
     c.remove("sync")
     return c
   })