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If you have both modelscale and modelscale_vec, moving the object resets the size to whatever the modelscale is set to, yet in the entitiy window modelscale_vec is still present.
If you remove modelscale at this point, modelscale_vec still has no effect until you change it's numbers. Just highlighting and pressing enter on the existing numbers to refresh is not enough.
Cannot reproduce, likely specific to 1.5 branch. In GtkRadiant 1.6, modelscale_vec overrides modelscale, though both values are kept. Works fine and as expected.