macOS: make conversation and main header draggable

ts/util/isWindowDragElement.ts

@@ -0,0 +1,21 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function isWindowDragElement(el: Readonly<Element>): boolean {
+  let currentEl: Element | null = el;
+  do {
+    const appRegion = getComputedStyle(currentEl).getPropertyValue(
+      '-webkit-app-region'
+    );
+    switch (appRegion) {
+      case 'no-drag':
+        return false;
+      case 'drag':
+        return true;
+      default:
+        currentEl = currentEl.parentElement;
+        break;
+    }
+  } while (currentEl);
+  return false;
+}