Adding comments on a pull request that has a large amount of difference between the source and target has a delay

Issue Summary

Typing in a comment box in a pull request is noticeably delayed when the number of displayed lines is large (1000+ lines).

There are two known ways to see the problem:

• A small diff on a large file where the context has been expanded
• The diff is large enough to show a warning

(Note: the original issue was for a file where the diff was large enough to show the "this diff is too big" but even for small diffs on large files - an example file had a 3-line change on a file with 1200 lines - typing in the comment is slow once the context is expanded)

Issue is observed: in Chrome, Firefox

Steps to Reproduce

1. Create new PR where the file changed is very large (1000 lines) - you only need to make a one-line change on the file
2. Open the diff view for the file.
3. Expand context on the file so it's showing all/most lines in the file (in cases where the change is at the beginning/end of the file, this only requires one click)
4. Add a comment to the PR diff

Expected Results

Characters typed in the comment appear without lag.

Actual Results

Characters typed in the comment only appear after several seconds.
It can take up to 10 seconds, depending on the PR diff.

Workaround

Currently there is no known workaround for this behaviour.
A workaround will be added here when available