Image darkening perceptively when Merge Visible is applied to editing layer stack.

​@L F Brown 

​@Stephen Marsh  is 100% correct - yes, pun intended ;-)

The reason this happens is that all blending and adjustment previews are calculated on the on-screen data, for performance reasons. When you’re zoomed out, that means a resampled and softened version, containing a lot of intermediate values that aren’t there in the full original pixel data.

When you flatten/commit, the adjustment is recalculated on the full data set, and so it may appear to “change”. But is’t not; the result is correct, it was the preview that was incorrect.

This is why you should always make it a habit to do a quick check at 100% first. Just hit ctrl+1.

For a “normal” photograph or a digital painting without noise this has no impact. It happens when you have very small pixel-level sharp transitions - transitions that are smoothed out when resampling for screen. Noise, starry night skies, line art with thin lines, that sort of thing - that’s when you need to look out.

Did the document have Smart Objects?  I have occasionally had issues when flattening such documents.  I can’t say, with hand  on heart, that I was viewing at 100% as per Stephens question.  What I have done to get round it though, was add a copy merged layer to the top of the stack (Shift Ctrl Alt E) before flattening.