copy button (#177)

- client.js only
- visual feedback with css
- uses navigator.clipboard.writeText
- adds the copy button on all PRE > CODE (or even PRE > SPAN) elements
- pass data-copy=off to disable

supersedes #138

public/client.js

@@ -334,3 +334,39 @@ function preventDoubleClick(event) {
 for (const summary of document.querySelectorAll("#observablehq-sidebar summary")) {
   summary.onmousedown = preventDoubleClick;
 }
+
+// copy code cells
+document.addEventListener("pointerover", ({target}) => {
+  if (typeof navigator?.clipboard?.writeText !== "function") return;
+  if (target.nodeName === "PRE" && !target.getAttribute("data-copy")) {
+    target.addEventListener("pointermove", move);
+    target.addEventListener("pointerleave", out);
+  }
+  function out() {
+    target.removeEventListener("pointermove", move);
+    target.removeEventListener("pointerleave", out);
+    target.removeEventListener("click", copy);
+    target.removeAttribute("data-copy");
+  }
+  function move({offsetX: x}) {
+    if (30 + x > parseInt(getComputedStyle(target).width)) {
+      if (!target.getAttribute("data-copy")) {
+        target.setAttribute("data-copy", "copy");
+        target.addEventListener("click", copy);
+      }
+    } else {
+      if (target.getAttribute("data-copy")) {
+        target.removeAttribute("data-copy");
+        target.removeEventListener("click", copy);
+      }
+    }
+  }
+  async function copy() {
+    try {
+      await navigator.clipboard.writeText(target.textContent);
+      target.setAttribute("data-copy", "copied");
+    } catch {
+      target.setAttribute("data-copy", "error");
+    }
+  }
+});

public/style.css

@@ -360,13 +360,7 @@ pre {
   overflow-x: auto;
 }
 
-pre .language-md::after,
-pre .language-tex::after,
-pre .language-dot::after,
-pre .language-mermaid::after,
-pre .language-js::after,
-pre .language-sh::after,
-pre .language-sql::after {
+pre code::after {
   position: absolute;
   right: 0;
   top: 0;
@@ -377,13 +371,17 @@ pre .language-sql::after {
   padding: 0.5rem 0.5rem 0.5rem 1rem;
 }
 
-pre .language-md::after { content: "md"; }
-pre .language-js::after { content: "js"; }
-pre .language-sh::after { content: "sh"; }
-pre .language-sql::after { content: "sql"; }
-pre .language-tex::after { content: "tex"; }
-pre .language-dot::after { content: "dot"; }
-pre .language-mermaid::after { content: "mermaid"; }
+pre code.language-md::after { content: "md"; }
+pre code.language-js::after { content: "js"; }
+pre code.language-sh::after { content: "sh"; }
+pre code.language-sql::after { content: "sql"; }
+pre code.language-tex::after { content: "tex"; }
+pre code.language-dot::after { content: "dot"; }
+pre code.language-mermaid::after { content: "mermaid"; }
+pre[data-copy="copy"] code::after { content: "copy"; }
+pre[data-copy="copied"] code::after { content: "copied"; }
+pre[data-copy="error"] code::after { content: "error"; }
+pre code::after { pointer-events: none; }
 
 input:not([type]),
 input[type="email"],