Why is my `HLine` `QFrame` invisible (inside my `QDockWidget`)?

[RokeJulianLockhart]

As aforestated at post/820286:

From what I see, the vertical line is a QFrame element with VLine settings.

Using stackoverflow.com/review/suggested-edits/36711790, that:

separator_line = PyQt6.QtWidgets.QFrame()
separator_line.setFrameShape(PyQt6.QtWidgets.QFrame.Shape.HLine)
separator_line.setFrameShadow(PyQt6.QtWidgets.QFrame.Shadow.Sunken)

...seems to render in the correct orientation for me:

1. Before

   

2. Afterward

   ...albeit invisibly (it should be between "Modify" and "Overflow"):

   

Any idea why it's invisible? I ask because although I expected that it would be a deficiency in Breeze (as improbable as that is for such a basic control in such a popular QStyle), it was invisible in Fusion too:

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If you need more code, I'll try to provide it. However, I can't provide all of it, because Akismet becomes angry.

[RokeJulianLockhart]

I'm stupid. It's because it was a 0-width HLine. Changed to VLine; worked.