This is a general issue with single pixel lines. As the tooltip moves, the border will occasionally get between pixels which results in that line becoming dimmer.
One way you can work around this is ensuring that 1) both the width and height are whole numbers and 2) the frame is positioned on whole pixels. Using floor() and/or ceil() can help in both cases.
Also, if you're not anchoring corner to corner, you may need to add .5.
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