Resolve "On master, inline diff view contains extra `+` and `-` signs"

app/assets/javascripts/diffs/store/utils.js

@@ -205,7 +205,8 @@ export function prepareDiffData(diffData) {
     if (file.highlightedDiffLines) {
       const linesLength = file.highlightedDiffLines.length;
       for (let u = 0; u < linesLength; u += 1) {
-        trimFirstCharOfLineContent(file.highlightedDiffLines[u]);
+        const line = file.highlightedDiffLines[u];
+        Object.assign(line, { ...trimFirstCharOfLineContent(line) });
       }
       showingLines += file.parallelDiffLines.length;
     }

changelogs/unreleased/51281-on-master-diff-view-contains-extra-and-signs.yml

+---
+title: Fixes double +/- on inline diff view
+merge_request: 21634
+author:
+type: fixed

spec/javascripts/diffs/store/utils_spec.js

@@ -231,9 +231,18 @@ describe('DiffsStoreUtils', () => {
       expect(firstParallelDiffLine.left).not.toHaveAttr('text');
       expect(firstParallelDiffLine.right.discussions.length).toBe(0);
       expect(firstParallelDiffLine.right).not.toHaveAttr('text');
-
-      expect(preparedDiff.diffFiles[0].highlightedDiffLines[0].discussions.length).toBe(0);
-      expect(preparedDiff.diffFiles[0].highlightedDiffLines[0]).not.toHaveAttr('text');
+      const firstParallelChar = firstParallelDiffLine.right.richText.charAt(0);
+      expect(firstParallelChar).not.toBe(' ');
+      expect(firstParallelChar).not.toBe('+');
+      expect(firstParallelChar).not.toBe('-');
+
+      const checkLine = preparedDiff.diffFiles[0].highlightedDiffLines[0];
+      expect(checkLine.discussions.length).toBe(0);
+      expect(checkLine).not.toHaveAttr('text');
+      const firstChar = checkLine.richText.charAt(0);
+      expect(firstChar).not.toBe(' ');
+      expect(firstChar).not.toBe('+');
+      expect(firstChar).not.toBe('-');
       expect(preparedDiff.diffFiles[0].renderIt).toBeTruthy();
       expect(preparedDiff.diffFiles[0].collapsed).toBeFalsy();