fix: improve output for very small visual diff percentage (#1779)

Author: WickyNilliams

Diff for: .changeset/curly-cameras-deliver.md

@@ -0,0 +1,5 @@
+---
+'@web/test-runner-visual-regression': patch
+---
+
+improve output in visual regression tests when very small differences from baseline

Diff for: packages/test-runner-visual-regression/src/visualDiffCommand.ts

@@ -104,11 +104,14 @@ export async function visualDiffCommand(
     await saveFailed();
   }
 
+  // if diff is suitably small, output raw value, otherwise to two decimal points.
+  // this avoids outputting a message like "New screenshot is 0.00% different"
+  const diffPercentageToDisplay =
+    diffPercentage < 0.005 ? diffPercentage : diffPercentage.toFixed(2);
+
   return {
     errorMessage: !passed
-      ? `Visual diff failed. New screenshot is ${diffPercentage.toFixed(
-          2,
-        )}% different.\nSee diff for details: ${diffFilePath}`
+      ? `Visual diff failed. New screenshot is ${diffPercentageToDisplay}% different.\nSee diff for details: ${diffFilePath}`
       : undefined,
     diffPercentage: -1,
     passed,