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When compositing is disabled, the oxygen theme makes separate windows blend together; This is especially apparent then there are no side-borders on windows.

This idea is that Oxygen should draw a dark outline around the window when compositing is disabled, or Kwin could use per-pixel operations to 50% darken a 1px border around windows; Either allowing users to more easily distinguish between windows.

Per-pixel operations to darken a 1px border around windows by 50% would be ideal, and I believe within a computational tolerance. Otherwise, the example below is showing just a dark-grey outline. It shows the same windows with compositing, sans, and with the outline idea.