QtWebDriver supports the following custom capabilities

• browserStartWindow
  QtWebDriver will search all existing top-level windows and attach to the one with the matching title.
  If * is specified then the first found window will be picked.
  See View Selection.
• browserClass
  QtWebDriver will create a window from the specified classname.
• loggingPrefs
  See LoggingPreferences JSON object for general usage, Logging for more details.
• maximize
  QtWebDriver will resize the window to its maximum size
• reuseUI
  If true, the starting QtWebDriver session will terminate any pre-existing session without closing the opened windows. e.g:

DesiredCapabilities cap = new DesiredCapabilities();
//specify reuseUI to have WebDriver terminate any previous session and reuse its windows
cap.setCapability("reuseUI", true);
 
//specify to select the first found window
cap.setCapability("browserStartWindow", "*");
wd = new RemoteWebDriver(new URL("https://localhost:9517"), cap);

• windowsize
  QtWebDriver will resize the window to the specified size, as a string (e.g. - "600, 600")
• windowposition
  QtWebDriver will position the window to the specified position, as a string (e.g. - "80, 60")