fix(web): keep diff file lists scrollable past expanded files (#6423)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: dominic-r

apps/web/src/components/diffs/StyledDiffCodeView.test.tsx

@@ -44,7 +44,7 @@ describe("StyledDiffCodeView", () => {
         diffHeaderHeight: 32,
         hunkSeparatorHeight: 24,
         paddingTop: 0,
-        paddingBottom: 0,
+        paddingBottom: 8,
       },
       layout: { paddingTop: 0, paddingBottom: 0, gap: 0 },
     });

apps/web/src/components/diffs/StyledDiffCodeView.tsx

@@ -304,11 +304,16 @@ export function StyledDiffCodeView<LAnnotation = undefined>({
           diffHeaderHeight: 32,
           hunkSeparatorHeight: 24,
           // Pierre uses its general file spacing as a fallback in expanded-file layout paths.
-          // Keep it zero alongside the explicit paddings or expanding the first file can
+          // Keep it zero alongside the explicit paddingTop or expanding the first file can
           // reintroduce the library's default 8px gap above its header.
           spacing: 0,
           paddingTop: 0,
-          paddingBottom: 0,
+          // Unlike the gap above, the 8px under a file's last line is painted
+          // unconditionally by Pierre's stylesheet (`--diffs-gap-fallback`), so the metric has
+          // to count it: at zero every expanded file's virtual height ran 8px short of its
+          // rendered height, and the end of the list sat past the reachable scroll range —
+          // one clipped file row per expanded file above it.
+          paddingBottom: 8,
         },
         layout: { paddingTop: 0, paddingBottom: 0, gap: 0 },
       }}