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Fix for this bug report on 6.4 will not be applied since we don't support 6.4 anymore. It will be fixed on other supported versions.
Reproduction steps:
1. Open the attached project Repro Project in the latest comment
2. Open the “/Assets/Scenes/SampleScene.unity” Scene
3. Enter a new "Fixed Resolution 1280x720" item in the Game view aspect drop-down
4. Select the 1280x720 aspect item
5. Resize the GameView so that around the rendered image Unity Editor displays dark gray borders
6. Enter the Play Mode
7. Wait until the Play Mode exits automatically
8. Observe the “/Captures/CaptureScreenshotAsTexture.png“
Expected result: No dark gray borders exist and looks identical to “/Captures/CaptureScreenshot.png“
Actual result: Dark Gray borders are present