Digital Research's Graphics Environment Manager - or GEM - was released for PCs and the Atari ST in 1985.
Unfortunately it was a liitle bit too close in look-and-feel to Apple's recently released OS on the Macintosh (and earlier the Lisa), leading to legal pressure to remove certain features.
This is the story of how GEM 1 ultimately ended up being by far the most feature-packed release as a result, and how this may well have lead to Apple and Microsoft becoming the dominant desktop platforms throughout the 90s and beyond.