From here:

https://answers.atlassian.com/questions/306181/diffing-branches-this-changeset-is-too-large-to-render-showing-the-first-1000-files

• Configure the page.max.changes property to be e.g. 50000
• Create a branch with a commit that changes more than 1000 files
• Compare that branch with another branch
• Open the Diff tab

Expected: Should show all changes
Actual: Shows only the first 1000 files with the message "This changeset is too large to render. Showing the first 1000 files."

Could reproduce this with the current Stash version.