fix #43; clear on fulfilled if no needed

Author: mbostock

src/runtime/define.ts

@@ -35,6 +35,7 @@ export function define(main: Module, state: DefineState, definition: Definition,
   const variables = state.variables;
   const v = main.variable(observer(state, definition), {shadow: {}});
   const vid = output ?? (outputs.length ? `cell ${id}` : null);
+  state.autoclear = true;
   if (inputs.includes("display") || inputs.includes("view")) {
     let displayVersion = -1; // the variable._version of currently-displayed values
     const vd = new (v.constructor as typeof Variable)(2, v._module);

src/runtime/display.ts

@@ -5,6 +5,8 @@ import {mapAssets} from "./stdlib/assets.js";
 export type DisplayState = {
   /** the HTML element in which to render this cell’s display */
   root: HTMLDivElement;
+  /** whether to clear on fulfilled */
+  autoclear?: boolean;
   /** for inspected values, any expanded paths; see getExpanded */
   expanded: (number[][] | undefined)[];
 };
@@ -42,6 +44,7 @@ function isDisplayable(value: unknown, root: HTMLDivElement): value is Node {
 }
 
 export function clear(state: DisplayState): void {
+  state.autoclear = false;
   state.expanded = Array.from(state.root.childNodes, getExpanded);
   while (state.root.lastChild) state.root.lastChild.remove();
 }
@@ -58,9 +61,11 @@ export function observe(state: DisplayState, {autodisplay, assets}: Definition)
     },
     fulfilled(value: unknown) {
       if (autodisplay) {
-        clear(state);
         if (assets && value instanceof Element) mapAssets(value, assets);
+        clear(state);
         display(state, value);
+      } else if (state.autoclear) {
+        clear(state);
       }
     },
     rejected(error: unknown) {