I further tested the LightBox and you are correct that when the LightBox window is opened the default keyboard shortcuts are prevented and this appear to be an issue in the control when the KeyBoardNavigation is enabled.
I will log this into our tracking system so that our dev team will take a look at this issue and will do their best to provide a fix as soon possible.
Meanwhile you could force the development tool to appear by right clicking on the light box and call the "InspectElement" option.
As a token of gratitude for sharing this issue you will find your Telerik points updated.
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