Popup Bug in WPF GridComboBox demo by Marco Zhou

I need to have a ComboBox with multiple columns and column headers. I found Marco Zhou's GridComboBox demo which I like because it's a pretty bare bones approach to doing so and it supports declarative column binding. I've found a couple minor "bugs" or lacking styles with it, but the one that has me stumped is when I bind the combobox to a DataView with a few thousand rows, the first time I open the drop down, the popup displays in the top left hand corner for a few seconds, then jumps down under the toggle button where it should be. I notice while its displaying in the top left, if I hover my mouse over the items its response lags. After it pops back into the correct position it responds quickly. So it seems like it is still loading/processing while it is in the top left and then updates its position once it's done. Almost like virtualization is not working. How can this be fixed?

http://www.cnblogs.com/Files/shev开发者_如何学Goa/GridComboBoxDemo.zip

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ComboBox is not virtualizing by default. It must be switched on manually and this is not done in the code you provided. Read about: VirtualizingStackPanel for ComboBoxes.