If you are having issues with the Merge Request Review Overview section not loading please do the following:

In the Project General Setting go to the Merge Request section, expand. Make a change - any change - and save. Revert the change and save.

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Choose a branch/tag (e.g. master ) or enter a commit (e.g. 4eedf23 ) to see what's changed or to create a merge request.
Changes are shown as if the source revision was being merged into the target revision.