1 files changed, 65 insertions, 0 deletions
diff --git a/browser/components/customizableui/ScrollbarSampler.jsm b/browser/components/customizableui/ScrollbarSampler.jsm
new file mode 100644
index 000000000..44736e4c4
--- /dev/null
+++ b/browser/components/customizableui/ScrollbarSampler.jsm
@@ -0,0 +1,65 @@
+/* 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";
+
+this.EXPORTED_SYMBOLS = ["ScrollbarSampler"];
+
+const {classes: Cc, interfaces: Ci, utils: Cu, results: Cr} = Components;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+var gSystemScrollbarWidth = null;
+
+this.ScrollbarSampler = {
+  getSystemScrollbarWidth: function() {
+    if (gSystemScrollbarWidth !== null) {
+      return Promise.resolve(gSystemScrollbarWidth);
+    }
+
+    return new Promise(resolve => {
+      this._sampleSystemScrollbarWidth().then(function(systemScrollbarWidth) {
+        gSystemScrollbarWidth = systemScrollbarWidth;
+        resolve(gSystemScrollbarWidth);
+      });
+    });
+  },
+
+  resetSystemScrollbarWidth: function() {
+    gSystemScrollbarWidth = null;
+  },
+
+  _sampleSystemScrollbarWidth: function() {
+    let hwin = Services.appShell.hiddenDOMWindow;
+    let hdoc = hwin.document.documentElement;
+    let iframe = hwin.document.createElementNS("http://www.w3.org/1999/xhtml",
+                                               "html:iframe");
+    iframe.setAttribute("srcdoc", '<body style="overflow-y: scroll"></body>');
+    hdoc.appendChild(iframe);
+
+    let cwindow = iframe.contentWindow;
+    let utils = cwindow.QueryInterface(Ci.nsIInterfaceRequestor)
+                       .getInterface(Ci.nsIDOMWindowUtils);
+
+    return new Promise(resolve => {
+      cwindow.addEventListener("load", function onLoad(aEvent) {
+        cwindow.removeEventListener("load", onLoad);
+        let sbWidth = {};
+        try {
+          utils.getScrollbarSize(true, sbWidth, {});
+        } catch (e) {
+          Cu.reportError("Could not sample scrollbar size: " + e + " -- " +
+                         e.stack);
+          sbWidth.value = 0;
+        }
+        // Minimum width of 10 so that we have enough padding:
+        sbWidth.value = Math.max(sbWidth.value, 10);
+        resolve(sbWidth.value);
+        iframe.remove();
+      });
+    });
+  }
+};
+Object.freeze(this.ScrollbarSampler);