fix tip flicker (observablehq#1826)

* closes observablehq#1805

* tests

* conditional postrender

---------

Co-authored-by: Mike Bostock <mbostock@gmail.com>

src/marks/tip.js

@@ -241,6 +241,7 @@ export class Tip extends Mark {
         text.setAttribute("y", `${+getLineOffset(a, text.childNodes.length, lineHeight).toFixed(6)}em`);
         text.setAttribute("transform", `translate(${getTextTranslate(a, m, r, w, h)})`);
       });
+      g.attr("visibility", null);
     }
 
     // Wait until the plot is inserted into the page so that we can use getBBox
@@ -250,8 +251,11 @@ export class Tip extends Mark {
     // this step. Perhaps this could be done synchronously; getting the
     // dimensions of the SVG is easy, and although accurate text metrics are
     // hard, we could use approximate heuristics.
-    if (svg.isConnected) Promise.resolve().then(postrender);
-    else if (typeof requestAnimationFrame !== "undefined") requestAnimationFrame(postrender);
+    if (index.length) {
+      g.attr("visibility", "hidden"); // hide until postrender
+      if (svg.isConnected) Promise.resolve().then(postrender);
+      else if (typeof requestAnimationFrame !== "undefined") requestAnimationFrame(postrender);
+    }
 
     return g.node();
   }