Replace empty placeholder diff row with colgroup

Removes the thin white gap at the top of the diff
code blocks which was added to preserve column sizes
when diff expansion rows would use `colspan` in the first row.

https://gitlab.com/gitlab-org/gitlab/merge_requests/17026

app/assets/javascripts/diffs/components/inline_diff_view.vue

@@ -45,12 +45,11 @@ export default {
     :data-commit-id="commitId"
     class="code diff-wrap-lines js-syntax-highlight text-file js-diff-inline-view"
   >
-    <!-- Need to insert an empty row to solve "table-layout:fixed" equal width when expansion row is the first line -->
-    <tr>
-      <td style="width: 50px;"></td>
-      <td style="width: 50px;"></td>
-      <td></td>
-    </tr>
+    <colgroup>
+      <col style="width: 50px;" />
+      <col style="width: 50px;" />
+      <col />
+    </colgroup>
     <tbody>
       <template v-for="(line, index) in diffLines">
         <inline-diff-expansion-row

app/assets/javascripts/diffs/components/parallel_diff_view.vue

@@ -45,13 +45,12 @@ export default {
     :data-commit-id="commitId"
     class="code diff-wrap-lines js-syntax-highlight text-file"
   >
-    <!-- Need to insert an empty row to solve "table-layout:fixed" equal width when expansion row is the first line -->
-    <tr>
-      <td style="width: 50px;"></td>
-      <td></td>
-      <td style="width: 50px;"></td>
-      <td></td>
-    </tr>
+    <colgroup>
+      <col style="width: 50px;" />
+      <col />
+      <col style="width: 50px;" />
+      <col />
+    </colgroup>
     <tbody>
       <template v-for="(line, index) in diffLines">
         <parallel-diff-expansion-row

changelogs/unreleased/32192-remove-gap-at-the-top-of-expandable-code-diff-blocks.yml

+---
+title: Remove thin white line at top of diff view code blocks
+merge_request: 17026
+author:
+type: fixed