LayoutTests/editing/selection/modal-dialog-select-paragraph.html

<dialog><p>This text should be selectable with the mouse.</p></dialog>
<div id="log"></div>
<script>
document.querySelector("dialog").showModal();

function selectParagraphWithMouse(p) {
    const rect = p.getBoundingClientRect();
    const x = rect.left;
    const y = rect.top + rect.height / 2;
    const textLength = p.textContent.length;
    eventSender.mouseMoveTo(x, y);
    eventSender.mouseDown();
    eventSender.leapForward(200);
    eventSender.mouseMoveTo(x + rect.width, y);
    eventSender.mouseUp();
}

if (window.eventSender) {
    testRunner.dumpAsText();

    const p = document.querySelector("p");
    selectParagraphWithMouse(p);

    const selection = getSelection().toString();
    if (selection == p.textContent)
        log.textContent = "PASS: all dialog text is selected";
    else
        log.textContent = "FAIL: unexpected selection, got: " + selection;
}
</script>