DockSettings exposes an option to make floating windows owned by the main window so they stay in front.
// Keep floating windows above the main window
DockSettings.UseOwnerForFloatingWindows = true;
// Via the app builder
using Dock.Settings;
AppBuilder.Configure<App>()
.UsePlatformDetect()
.UseOwnerForFloatingWindows();
When enabled Dock sets the main window as the owner for floating windows, preventing them from appearing behind it. Disable this if you want windows to be independent.