This is using latest Firefox with geckodriver, and it passes with Ruby bindings directly to geckodriver. It was also passing the last time I ran these specs via remote server.
-> GET session/9900327b-8b39-4a5b-8bd8-51e6a2d8b524/window/size
<- {"state":"invalid argument","sessionId":"9900327b-8b39-4a5b-8bd8-51e6a2d8b524","hCode":1366879402,"value":{"localizedMessage":null,"cause":null,"stackTrace":[{"fileName":"RemoteWebDriver.java","nativeMethod":false,"methodName":"getSize","className":"org.openqa.selenium.remote.RemoteWebDriver$RemoteWebDriverOptions$RemoteWindow","hCode":645985369,"lineNumber":862,"class":"java.lang.StackTraceElement"},{"fileName":"EventFiringWebDriver.java","nativeMethod":false,"methodName":"getSize","className":"org.openqa.selenium.support.events.EventFiringWebDriver$EventFiringWindow","hCode":845010198,"lineNumber":632,"class":"java.lang.StackTraceElement"},{"fileName":"GetWindowSize.java","nativeMethod":false,"methodName":"call","className":"org.openqa.selenium.remote.server.handler.GetWindowSize","hCode":-1883499878,"lineNumber":31,"class":"java.lang.StackTraceElement"},{"fileName":"GetWindowSize.java","nativeMethod":false,"methodName":"call","className":"org.openqa.selenium.remote.server.handler.GetWindowSize","hCode":-1883499886,"lineNumber":23,"class":"java.lang.StackTraceElement"},{"fileName":"FutureTask.java","nativeMethod":false,"methodName":"run","className":"java.util.concurrent.FutureTask","hCode":-820313554,"lineNumber":266,"class":"java.lang.StackTraceElement"},{"fileName":"DefaultSession.java","nativeMethod":false,"methodName":"run","className":"org.openqa.selenium.remote.server.DefaultSession$1","hCode":-255143467,"lineNumber":176,"class":"java.lang.StackTraceElement"},{"fileName":"ThreadPoolExecutor.java","nativeMethod":false,"methodName":"runWorker","className":"java.util.concurrent.ThreadPoolExecutor","hCode":-1285150184,"lineNumber":1142,"class":"java.lang.StackTraceElement"},{"fileName":"ThreadPoolExecutor.java","nativeMethod":false,"methodName":"run","className":"java.util.concurrent.ThreadPoolExecutor$Worker","hCode":-242190645,"lineNumber":617,"class":"java.lang.StackTraceElement"},{"fileName":"Thread.java","nativeMethod":false,"methodName":"run","className":"java.lang.Thread","hCode":-82946320,"lineNumber":745,"class":"java.lang.StackTraceElement"}],"suppressed":[],"message":null,"hCode":92059481,"class":"java.lang.NullPointerException","screen":null},"class":"org.openqa.selenium.remote.Response","status":13}
This is using latest Firefox with geckodriver, and it passes with Ruby bindings directly to geckodriver. It was also passing the last time I ran these specs via remote server.
Server stacktrace: https://gist.github.com/titusfortner/fc0bec840a1e0653effcfcdb4e752cf3