From 33058f2b292b3a581333bdfb21b8f671898c5060 Mon Sep 17 00:00:00 2001
From: Peter Bengtsson <mail@peterbe.com>
Date: Tue, 8 Dec 2020 14:40:17 -0500
Subject: initial commit

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+---
+title: Window.screenTop
+slug: Web/API/Window/screenTop
+translation_of: Web/API/Window/screenTop
+---
+<p>{{APIRef}}</p>
+
+<p><code><strong>Window.screenTop</strong></code> 只读属性返回垂直距离，单位是CSS 像素, 从用户浏览器的上边界到屏幕最顶端。</p>
+
+<div class="blockIndicator note">
+<p><strong>Note</strong>: <code>screenTop</code> is an alias of the older {{domxref("Window.screenY")}} property. <code>screenTop</code> was originally supported only in IE but was introduced everywhere due to popularity.</p>
+</div>
+
+<h2 id="Syntax" name="Syntax">语法</h2>
+
+<pre class="syntaxbox"><em>topWindowPos</em> = window.screenTop
+</pre>
+
+<h3 id="返回值">返回值</h3>
+
+<p>A number equal to the number of CSS pixels from the top edge of the browser viewport to the  top edge of the screen.</p>
+
+<h2 id="Specifications" name="Specifications">例子</h2>
+
+<p>In our <a href="https://mdn.github.io/dom-examples/screenleft-screentop/">screenleft-screentop</a> example, you'll see a canvas onto which has been drawn a circle. In this example we are using <code>screenLeft</code>/<code>screenTop</code> plus {{domxref("Window.requestAnimationFrame()")}} to constantly redraw the circle in the same physical position on the screen, even if the window position is moved.</p>
+
+<pre class="brush: js">initialLeft = window.screenLeft + canvasElem.offsetLeft;
+initialTop = window.screenTop + canvasElem.offsetTop;
+
+function positionElem() {
+  let newLeft = window.screenLeft + canvasElem.offsetLeft;
+  let newTop = window.screenTop + canvasElem.offsetTop;
+
+  let leftUpdate = initialLeft - newLeft;
+  let topUpdate = initialTop - newTop;
+
+  ctx.fillStyle = 'rgb(0, 0, 0)';
+  ctx.fillRect(0, 0, width, height);
+  ctx.fillStyle = 'rgb(0, 0, 255)';
+  ctx.beginPath();
+  ctx.arc(leftUpdate + (width/2), topUpdate + (height/2) + 35, 50, degToRad(0), degToRad(360), false);
+  ctx.fill();
+
+  pElem.textContent = 'Window.screenLeft: ' + window.screenLeft + ', Window.screenTop: ' + window.screenTop;
+
+  window.requestAnimationFrame(positionElem);
+}
+
+window.requestAnimationFrame(positionElem);</pre>
+
+<p>Also in the code we include a snippet that detects whether <code>screenLeft</code> is supported, and if not, polyfills in <code>screenLeft</code>/<code>screenTop</code> using {{domxref("Window.screenX")}}/{{domxref("Window.screenY")}}.</p>
+
+<pre class="brush: js">if(!window.screenLeft) {
+  window.screenLeft = window.screenX;
+  window.screenTop = window.screenY;
+}</pre>
+
+<h2 id="Specifications" name="Specifications">说明</h2>
+
+<table class="standard-table" style="height: 49px; width: 1000px;">
+ <thead>
+  <tr>
+   <th scope="col">Specification</th>
+   <th scope="col">Status</th>
+   <th scope="col">Comment</th>
+  </tr>
+ </thead>
+ <tbody>
+  <tr>
+   <td>{{ SpecName('CSSOM View', '#dom-window-screeny', 'Window.screenTop') }}</td>
+   <td>{{ Spec2('CSSOM View') }}</td>
+   <td>Initial definition.</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+
+
+<p>{{Compat("api.Window.screenTop")}}</p>
+
+<h2 id="参考">参考</h2>
+
+<ul>
+ <li>{{domxref("window.screenLeft")}}</li>
+ <li>{{domxref("Window.screenY")}}</li>
+</ul>
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