Erroneous surface coloration shows position of elements that were "differenced" out. about openscad

[jordanbrown0]

See also #4286 and many others (#4442, #4780, #4546, #4335, et cetera).

The simple answer is that yes, the previewer gets Z-fighting artifacts any time that a face of a negative object aligns with a face of a positive object. There can be similar issues when faces that have been subtracted away align with positive faces of a different color.

Here's the FAQ entry: https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/FAQ#What_are_those_strange_flickering_artifacts_in_the_preview?

There are roughly two strategies for addressing this problem:

• Arrange that those cases don't happen. The major case there is where you are subtracting a negative object from a positive one and cutting into its surface. Arrange that the negative object extends outside the positive object. (But that doesn't directly help your particular scenario.)
• Wrap strategic pieces with render(), which forces OpenSCAD to fully evaluate the geometry of that subassembly.

In your particular case, wrapping the difference() in ikea() in a render() appears to address the problem.

...
module ikea(r = 5, h = 3) {
   render() difference() {
                union() {
...

Why does it do this, and why doesn't somebody fix it? Because the previewer does black magic to get its performance, and it's a wonder that it works as well as it does, and nobody has a better answer. Recent work to dramatically improve render performance may provide a viable replacement, though it's still slower than the previewer and more limited in other ways like color and transparency support.