problems :
- game scene was not as shown in the GDD
- problem when moving tilemap (the placeholders didn't follow)
- possible to place units outside the game area
solutions :
- try to make the game scene as close to the gdd as possible
- don't move the tilemap : move the camera instead (this keeps the world position of tilemap intact)
- there was a logic bug in the DraggablePlaceholder. It ain't anymore
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