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Please try to reset your Viewer's settings and check if this solves the problem. You can do this from Main menu -> Edit -> Reset all settings to defaults.
Please note that resetting your settings will erase all custom settings that you might have. We recommend backing up your settings prior to resetting them from Edit -> Export all Settings to data file.
If the reset fails to solve the problem, you can restore your settings from Edit -> Import all Settings from data file.