[HTML]
<style>
textarea-copy.one {
	width: 300px;
	height: 100px;
	border: solid 1px #4ad;
}
textarea-copy.two {
	width: 400px;
	height: 100px;
	border-style: dashed;
	border-width: 0px 2px 2px 2px;
	border-color: #d4a;
	background-color: #fee;
	padding: 5px 10px;
	color: #333;
}
</style>

<textarea-copy class="one" alt="タイトル文字列1"></textarea-copy>

<textarea-copy class="two" alt="タイトル文字列2">ウェブコンポ・・・ます。</textarea-copy>

<script defer src="./js/ce_textarea_copy.js"></script>
		

/**
 * コピー機能付きtextarea
 *
 * @author ao-system, Inc.
 * @date 2024-03-02
 *
 * e.g.
 * <textarea-copy alt="タイトル文字列"></textarea-copy>
 * <script defer src="ce_textarea_copy.js"></script>
 *
 */

(() => {
	'use strict';
	class TextareaCopy {
		#constProperties = [
			{	prop: 'svgPath',
				value: '<path d="M2.77,10.45l-1.5,1.5V5.17c0-2.49,2.02-4.51,4.51-4.51h6.78l-1.5,1.5H5.78c-1.66,0-3.01,1.35-3.01,3.01V10.45z"/><path class="icon" d="M12.18,15.34H5.84c-1.3,0-2.35-1.05-2.35-2.35V5.15c0-1.3,1.05-2.35,2.35-2.35h6.34c1.3,0,2.35,1.05,2.35,2.35v7.84C14.53,14.29,13.48,15.34,12.18,15.34z M5.84,4.3c-0.47,0-0.85,0.38-0.85,0.85v7.84c0,0.47,0.38,0.85,0.85,0.85h6.34c0.47,0,0.85-0.38,0.85-0.85V5.15c0-0.47-0.38-0.85-0.85-0.85H5.84z"/>',
			},
			{	prop: 'style',
				value: `
					:host > div { /*wrapper*/
						--color-bar-fore: #fff;
						width: 100%;
						position: relative;
						> div {	/*bar*/
							width: 100%;
							padding: 2px 5px;
							box-sizing: border-box;
							display: flex;
							justify-content: space-between;
							align-items: center;
							column-gap: 5px;
							> div {		/*alt text*/
								font-size: 14px;
								color: var(--color-bar-fore);
								flex-grow: 1;
							}
							> span {	/*sign*/
								font-size: 14px;
								color: var(--color-bar-fore);
							}
							> svg {		/*copy*/
								fill: var(--color-bar-fore);
								opacity: 0.5;
								cursor: pointer;
								&:hover {
									opacity: 1;
								}
							}
						}
						> textarea {
							box-sizing: border-box;
						}
					}
				`,
			},
			{	prop: 'styleElm',
				value: document.createElement('style'),
			},
			{	prop: 'wrapElm',
				value: document.createElement('div'),
			},
			{	prop: 'barElm',
				value: document.createElement('div'),
			},
			{	prop: 'textareaElm',
				value: document.createElement('textarea'),
			},
		];
		constructor() {
			//class内でwritable:falseの定数を作成する方法
			this.#constProperties.forEach((obj) => Object.defineProperty(this, obj.prop, {value:obj.value, writable:false}));
		}
		#clipboardCopy() {
			return new Promise((resolve, reject) => {
				try {
					navigator.clipboard.writeText(this.textareaElm.value).then(() => {
						resolve('Copied!');
					}, () => {
						resolve('Could not copy');
					});
				} catch(e) {
					resolve('HTTPS required');
				}
			});
		}
		//styleを作成
		#styleElement() {
			this.styleElm.textContent = this.style;
		}
		//barを作成
		#barElement(elementRef) {
			const altElm = document.createElement('div');
			altElm.textContent = elementRef.getAttribute('alt') || '';
			this.barElm.appendChild(altElm);
			//
			const signElm = document.createElement('span');
			this.barElm.appendChild(signElm);
			//
			const svgElm = document.createElementNS('http://www.w3.org/2000/svg','svg');
			svgElm.setAttribute('width',16);
			svgElm.setAttribute('height',16);
			svgElm.innerHTML = this.svgPath;
			svgElm.addEventListener('click', async () => {
				signElm.textContent = await this.#clipboardCopy();
				setTimeout(() => { signElm.textContent = ''; },3000);
			});
			this.barElm.appendChild(svgElm);
			this.wrapElm.appendChild(this.barElm);
		}
		//textareaを作成
		#textareaElement(elementRef) {
			this.textareaElm.value = elementRef.innerHTML;
			this.wrapElm.appendChild(this.textareaElm);
		}
		//shadowDOMにappend
		#attachShadow(elementRef,styleElm,wrapElm) {
			const shadowClosed = elementRef.attachShadow({mode:'closed'});
			shadowClosed.innerHTML = '';
			shadowClosed.appendChild(this.styleElm);
			shadowClosed.appendChild(this.wrapElm);
		}
		//textareaのスタイル変更を監視
		#observeTextarea() {
			const observer = new MutationObserver((mutationsList, observer) => {
				for(const mutation of mutationsList) {
					if (mutation.type === 'attributes' && mutation.attributeName === 'style') {
						//textareaのwidthが変化したらwrapperのwidthに適用
						this.wrapElm.style.width = this.textareaElm.style.width;
					}
				}
			});
			observer.observe(this.textareaElm, { attributes:true, attributeFilter:['style'] });
		}
		//textarea-copyのスタイルを取得してshadowDOM内のエレメントに適用する
		#applyStyle(elementRef) {
			const computedStyle = window.getComputedStyle(elementRef);
			for (let i = 0; i < computedStyle.length; i++) {
			    const propertyName = computedStyle[i];
			    const propertyValue = computedStyle.getPropertyValue(propertyName);
				if (['box-sizing','resize','white-space-collapse'].includes(propertyName) == false) {	//これらを除く
				    this.textareaElm.style.setProperty(propertyName, propertyValue);
				}
			}
			//textarea-copyのborderTopColorをshadow内barのbackgroundColorに適用
			this.barElm.style.backgroundColor = computedStyle.borderTopColor;
			//textarea-copyのborderTopLeftRadius,borderTopRightRadiusをbarのborderTopLeftRadius,borderTopRightRadiusに適用
			this.barElm.style.setProperty('border-top-left-radius',computedStyle.borderTopLeftRadius);
			this.barElm.style.setProperty('border-top-right-radius',computedStyle.borderTopRightRadius);
			this.textareaElm.style.borderTopLeftRadius = 0;
			this.textareaElm.style.borderTopRightRadius = 0;
			//textareaのwidthをwrapperのwidthに適用
			this.wrapElm.style.width = this.textareaElm.clientWidth + 'px';
			//textarea-copyのスタイルを削除。<textarea-copy/>を非表示にする為
			elementRef.style.border = 'none';
			elementRef.style.padding = 0;
		}
		render(elementRef) {
			this.#styleElement();
			this.#barElement(elementRef);
			this.#textareaElement(elementRef);
			this.#attachShadow(elementRef);
			this.#observeTextarea();
			this.#applyStyle(elementRef);
		}
	}
	customElements.define('textarea-copy',
		class extends HTMLElement {
			constructor() {
				super();
				(new TextareaCopy()).render(this);
			}
		}
	);
})();