test_paste_clipboard_change.html

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<!doctype html>

<html>

<head>

  <title>Test for clipboard content changes during the paste event handler</title>

  <script src="/tests/SimpleTest/SimpleTest.js"></script>

  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>

</head>

<body>

<input type="text">

<div contenteditable="true"></div>

<script>

document.querySelectorAll("input, div[contenteditable]").forEach((element) => {

  add_task(async () => {

    info("test for " + element);

    info("Waiting for initializing clipboard...");

    await SimpleTest.promiseClipboardChange(

      "Original text",

      () => SpecialPowers.clipboardCopyString("Original text")

);

    element.focus();

    element.addEventListener("paste", () => {

      // Overwrite the clipboard content.

      SpecialPowers.clipboardCopyString("New text");

    }, { once: true });

    SpecialPowers.doCommand(window, "cmd_paste");

    // Check content.

    is(element.isContentEditable ? element.textContent : element.value, "New text", "Check paste content");

});

})

</script>

</body>

</html>