fixed sync alignment with unequal elements

fixes #21

src/core/draw/sequence.typ

@@ -394,6 +394,11 @@
   set-ctx(c => {
     c.y = end-info.y
     c.lifelines = ctx.lifelines
+    c.last-drawn = (
+      type: "seq",
+      start-info: start-info,
+      end-info: end-info
+    )
     return c
   })
 })

src/core/draw/sync.typ

@@ -1,11 +1,17 @@
 #import "/src/core/utils.typ": get-ctx, is-elmt, set-ctx
+#import "/src/cetz.typ": draw
 
 #let render(sync) = get-ctx(ctx => {
   set-ctx(c => {
-    c.sync-ys = ()
+    c.sync = (
+      bottoms: (),
+      starts: (),
+      y: ctx.y
+    )
     return c
   })
 
+  // Compute heights
   for e in sync.elmts {
     assert(is-elmt(e), message: "Sync element can only contain chronos elements, found " + repr(e))
     assert(
@@ -17,16 +23,34 @@
       c.y = ctx.y
       return c
     })
+    draw.hide({
       (e.draw)(e)
+    })
     set-ctx(c => {
-      c.sync-ys.push(c.y)
+      c.sync.starts.push(c.last-drawn.start-info.y)
+      c.sync.bottoms.push(c.y)
       return c
     })
   }
 
   set-ctx(c => {
-    c.y = calc.min(..c.sync-ys)
-    c.remove("sync-ys")
+    c.sync-y = calc.min(..c.sync.starts)
+    return c
+  })
+
+  // 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
+      return c
+    })
+    (e.draw)(e)
+  }
+
+  set-ctx(c => {
+    c.y = calc.min(..c.sync.bottoms)
+    c.remove("sync")
     return c
   })
 })

tests/sequence/sync/.gitignore

@@ -0,0 +1,4 @@
+# generated by tytanic, do not edit
+
+diff/**
+out/**

tests/sequence/sync/test.typ

@@ -0,0 +1,37 @@
+#set page(width: auto, height: auto)
+#import "/src/lib.typ": *
+
+#diagram({
+  _par("a")
+  _par("b")
+  _par("c")
+  _sync({
+    _seq("b", "a")
+    _seq("b", "c")
+  })
+  _gap()
+  _sync({
+    _seq("b", "a", comment: [Comment])
+    _seq("b", "c")
+  })
+  _gap()
+  _sync({
+    _seq("b", "a")
+    _seq("b", "c", comment: [Comment])
+  })
+  _gap()
+  _sync({
+    _seq("b", "a", comment: [Two\ lines])
+    _seq("b", "c", comment: [Comment])
+  })
+
+  _sync({
+    _seq("b", "a")
+    _seq("b", "c", slant: 10)
+  })
+
+  _sync({
+    _seq("b", "a")
+    _seq("b", "b")
+  })
+})