Indesign v18.4

Steps:
- make a rectangular frame, give it a background color
- make a polygon the same color as the page background, that intersects the edge of the frame
- cut and paste-into the polygon into the frame

Expected
the combination perfectly simulates the appearance of subtracting the polygon from the rectangular frame, but in a way that preserves editability.

Actual
works imperfectly: a tiny sliver of the background is visible on the edge of the rectangular frame. this very thin line gets thickened by the "enhance thin lines" function on adobe reader (but is still visible without it, if you zoom in). that's a problem because I can't exactly go ask my end audience to go turn that off for optimal viewing.

screenshots
first shows the paths of the frame and the polygon, second shows end result: a thin line visible where it shouldn't be that persists when exporting to pdf.