Fixes an issue where tab titles were not truncating when characters used 4 bytes for encoding (#2254)

* Fixes an issue where tab titles were not truncating when characters used 4 bytes for encoding.

* Changed substringUtf method to be more accurate and added comments

src/Utils/StringUtils.test.ts

@@ -26,5 +26,22 @@ describe("StringUtils", () => {
       const transformedString: string | undefined = StringUtils.stripSpacesFromString("");
       expect(transformedString).toBe("");
     });
+
+    it("should return the right number of characters regardless of bytes used per character", () => {
+      // Tried to use a sample of characters across the range for each of the individual byte lengths
+      const ascii = "!,n~!,n~!,n~";
+      const twoByteCharacters = "Āā߿܀Āā߿܀Āā߿܀";
+      const threeByteCharacters = "ࠀ倀ꀀࠀ倀ꀀࠀ倀ꀀ";
+      const fourByteCharacters = "𐀀𐔀𐨀𐿶𐀀𐔀𐨀𐿶𐀀𐔀𐨀𐿶";
+      // Used a random character generator for each of the different byte-lengths of characters for the mixed tests
+      const mixedByteSizes = "Yח䙶𫶾eԚ疿𱺿]߉ꗫ𢆤*ɉ貸𪡑";
+
+      expect(StringUtils.substringUtf(ascii, 0, 5)).toBe("!,n~!");
+      expect(StringUtils.substringUtf(twoByteCharacters, 0, 5)).toBe("Āā߿܀Ā");
+      expect(StringUtils.substringUtf(threeByteCharacters, 0, 5)).toBe("ࠀ倀ꀀࠀ");
+      expect(StringUtils.substringUtf(fourByteCharacters, 0, 5)).toBe("𐀀𐔀𐨀𐿶𐀀");
+      expect(StringUtils.substringUtf(mixedByteSizes, 0, 5)).toBe("Yח䙶𫶾e");
+      expect(StringUtils.substringUtf(mixedByteSizes, 4, 4)).toBe("eԚ疿𱺿");
+    });
   });
 });