Hi, i have the following lines, which successfully open a new Browser Window and shows the desired UI.
BrowserWindowOpener opener = new BrowserWindowOpener(PopupUI.class);
opener.setFeatures("height=768,width=1024,resizable=no");
Button button = new Button("Pop It Up");
opener.extend(button);
But still, the window can be resized. How can i prevent this?
Depending on browser settings that might not be possible at all. If you check https://developer.mozilla.org/en-US/docs/Web/API/Window/open#Position_and_size_features
It says “Starting with version 1.4, Mozilla-based browsers have a window resizing grippy at the right end of the status bar, this ensures that users can resize the browser window even if the web author requested this secondary window to be non-resizable.” and “Mozilla and Firefox users can force new windows to be easily resizable by setting dom.disable_window_open_feature.resizable to true in about:config or in their user.js file.”
You can however try this https://stackoverflow.com/questions/10610899/disable-browser-window-resize
and add a simple JS file to you PopupUI via https://vaadin.com/api/8.4.1/com/vaadin/annotations/JavaScript.html
HTH,
Ronny
Thank you, that kinda worked 
TBH, i am not using a clean form of Vaadin, so i had to redesign it partial, but it worked.