1 files changed, 52 insertions, 0 deletions
diff --git a/devtools/client/shared/scroll.js b/devtools/client/shared/scroll.js
new file mode 100644
index 000000000..ee591e014
--- /dev/null
+++ b/devtools/client/shared/scroll.js
@@ -0,0 +1,52 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * Scroll the document so that the element "elem" appears in the viewport.
+ *
+ * @param {DOMNode} elem
+ *        The element that needs to appear in the viewport.
+ * @param {Boolean} centered
+ *        true if you want it centered, false if you want it to appear on the
+ *        top of the viewport. It is true by default, and that is usually what
+ *        you want.
+ */
+function scrollIntoViewIfNeeded(elem, centered = true) {
+  let win = elem.ownerDocument.defaultView;
+  let clientRect = elem.getBoundingClientRect();
+
+  // The following are always from the {top, bottom}
+  // of the viewport, to the {top, …} of the box.
+  // Think of them as geometrical vectors, it helps.
+  // The origin is at the top left.
+
+  let topToBottom = clientRect.bottom;
+  let bottomToTop = clientRect.top - win.innerHeight;
+  // We allow one translation on the y axis.
+  let yAllowed = true;
+
+  // Whatever `centered` is, the behavior is the same if the box is
+  // (even partially) visible.
+  if ((topToBottom > 0 || !centered) && topToBottom <= elem.offsetHeight) {
+    win.scrollBy(0, topToBottom - elem.offsetHeight);
+    yAllowed = false;
+  } else if ((bottomToTop < 0 || !centered) &&
+             bottomToTop >= -elem.offsetHeight) {
+    win.scrollBy(0, bottomToTop + elem.offsetHeight);
+    yAllowed = false;
+  }
+
+  // If we want it centered, and the box is completely hidden,
+  // then we center it explicitly.
+  if (centered) {
+    if (yAllowed && (topToBottom <= 0 || bottomToTop >= 0)) {
+      win.scroll(win.scrollX,
+                 win.scrollY + clientRect.top
+                 - (win.innerHeight - elem.offsetHeight) / 2);
+    }
+  }
+}
+exports.scrollIntoViewIfNeeded = scrollIntoViewIfNeeded;