Try comparing <https://bugs.webkit.org/attachment.cgi?id=409243&action=diff> against `409068 - Patch` or `408943 - Patch`. I also often see "Warning: this difference between two patches may show things in the wrong places due to a limitation in Bugzilla when comparing patches with different sets of files.", such as when comparing <https://bugs.webkit.org/attachment.cgi?id=409243&action=diff> against `409313 - Patch`. I use the "Diff" link/tool to quickly/easily see what's changed between two patches so I can see just what's changed. This is especially useful for larger patches as in those cases it can be even harder to pick out the differences. When this tool doesn't work I'm left to downloading the before and after patches, copying them as plain text, pasting them into a separate diff tool (e.g. <https://www.diffchecker.com>), and then looking at the results, which is far from an ideal/quick process :(
<rdar://problem/69758647>
just happened again with <https://bugs.webkit.org/attachment.cgi?oldid=410255&action=interdiff&newid=410274&headers=1> :(