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• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

On android, there are cases when large images used as backgrounds lose their aspect ratio.
That is due to hardcoded limits in Fetcher class that attempt to downscale bitmaps to device screen bounds at certain cases.
What I mentioned above is an old feature that's implemented since old tns-core-widgets repo.
See NativeScript/tns-core-modules-widgets#115

Here is an example of how a large image used as a background with size contain looks in both platforms.
You'll notice that the android version upscales vertically.

What is the new behavior?

From now on, background fetched images will keep aspect ratio by default.

Result