Code is executing before a control is visually drawn to the screen stopping it ever getting drawn

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C#

WPF

I have a window with a combobox and a few other controls on and the combobox loads data from an xml file into labels to show to the user. The xml that is loaded changes depending on the selected index of the combobox. I also have a user control that I have made to look like a splash screen but it is for long operations so it has an image of a compass (don't ask why) and text to say 'Loading . . .'. Whenever the user selects an item in the combobox I want the usercontrol to become visible over the whole of the window so it covers it (it is already in place so the visibility just needs to be changed). Then the operation will complete and the usercontrol will become hidden.

It seems simple to do and I am currently changing the visibility on the selectionchanged event of the combobox but that it also where the large xml file is read. The problem I am having is that the code is executed fine but it does not have enough time to draw the usercontrol when the visibility is changed. SO it changes the visibility and thinks that it is then visible on the screen and moves on to the file reading when in fact the usercontrol's property is set to visible but that actual graphics has not been drawn to the screen so it then removes the purpose of having that waiting screen because it is never drawn. How can I avoid this so when the index is changes, the usercontrol is drawn to the screen before it moves on? Telling the thread to sleep would not work because it would be pausing the thread after it thinks it is drawn so it would only delay the finishing of the xml reading. I hope I have not explained it too much, thank you.