Properly detect scrollbar gutter

As a rule, instead of hard-coding a behavior on specific platforms we
should do dynamic detection.

This commit moves away from always hiding scrollbars on Android and iOS
and instead detects the rendered width of scrollbars in the browser.

kasmweb/app/ui.js

@@ -22,7 +22,7 @@ window.addEventListener("load", function() {
 
 import * as Log from '../core/util/logging.js';
 import _, { l10n } from './localization.js';
-import { isTouchDevice, isSafari, isIOS, isAndroid, dragThreshold }
+import { isTouchDevice, isSafari, hasScrollbarGutter, dragThreshold }
     from '../core/util/browser.js';
 import { setCapture, getPointerEvent } from '../core/util/events.js';
 import KeyTable from "../core/input/keysym.js";
@@ -1618,8 +1618,9 @@ const UI = {
             // Can't be clipping if viewport is scaled to fit
             UI.forceSetting('view_clip', false);
             UI.rfb.clipViewport  = false;
-        } else if (isIOS() || isAndroid()) {
-            // iOS and Android usually have shit scrollbars
+        } else if (!hasScrollbarGutter) {
+            // Some platforms have scrollbars that are difficult
+            // to use in our case, so we always use our own panning
             UI.forceSetting('view_clip', true);
             UI.rfb.clipViewport = true;
         } else {