Skip whitespace removal for multiline pastes (#13698)

## Summary of the Pull Request
Skips whitespace removal if pasted string is multiline

## PR Checklist
* [x] Closes #12387
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Tests added/passed
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

## Validation Steps Performed
Tests passed
The last command in multiline paste now executes

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -2163,18 +2163,12 @@ namespace winrt::TerminalApp::implementation
 
             if (_settings.GlobalSettings().TrimPaste())
             {
-                std::wstring_view textView{ text };
-                const auto pos = textView.find_last_not_of(L"\t\n\v\f\r ");
-                if (pos == textView.npos)
+                text = { Utils::TrimPaste(text) };
+                if (text.empty())
                 {
                     // Text is all white space, nothing to paste
                     co_return;
                 }
-                else if (const auto toRemove = textView.size() - 1 - pos; toRemove > 0)
-                {
-                    textView.remove_suffix(toRemove);
-                    text = { textView };
-                }
             }
 
             // If the requesting terminal is in bracketed paste mode, then we don't need to warn about a multi-line paste.

src/types/inc/utils.hpp

@@ -106,4 +106,9 @@ namespace Microsoft::Console::Utils
     std::tuple<std::wstring, std::wstring> MangleStartingDirectoryForWSL(std::wstring_view commandLine,
                                                                          std::wstring_view startingDirectory);
 
+    // Similar to MangleStartingDirectoryForWSL, this function is only ever used
+    // in TerminalPage::_PasteFromClipboardHandler, but putting it here makes
+    // testing easier.
+    std::wstring_view TrimPaste(std::wstring_view textView) noexcept;
+
 }

src/types/ut_types/UtilsTests.cpp

@@ -30,6 +30,9 @@ class UtilsTests
     TEST_METHOD(TestMangleWSLPaths);
 #endif
 
+    TEST_METHOD(TestTrimTrailingWhitespace);
+    TEST_METHOD(TestDontTrimTrailingWhitespace);
+
     void _VerifyXTermColorResult(const std::wstring_view wstr, DWORD colorValue);
     void _VerifyXTermColorInvalid(const std::wstring_view wstr);
 };
@@ -505,3 +508,41 @@ void UtilsTests::TestMangleWSLPaths()
     }
 }
 #endif
+
+void UtilsTests::TestTrimTrailingWhitespace()
+{
+    // Continue on failures
+    const WEX::TestExecution::DisableVerifyExceptions disableExceptionsScope;
+
+    // Tests for GH #11473
+    VERIFY_ARE_EQUAL(L"Foo", TrimPaste(L"Foo   "));
+    VERIFY_ARE_EQUAL(L"Foo", TrimPaste(L"Foo\n"));
+    VERIFY_ARE_EQUAL(L"Foo", TrimPaste(L"Foo\n\n"));
+    VERIFY_ARE_EQUAL(L"Foo", TrimPaste(L"Foo\r\n"));
+    VERIFY_ARE_EQUAL(L"Foo Bar", TrimPaste(L"Foo Bar\n"));
+    VERIFY_ARE_EQUAL(L"Foo\tBar", TrimPaste(L"Foo\tBar\n"));
+
+    VERIFY_ARE_EQUAL(L"Foo Bar", TrimPaste(L"Foo Bar\t"), L"Trim when there is a tab at the end.");
+    VERIFY_ARE_EQUAL(L"Foo Bar", TrimPaste(L"Foo Bar\t\t"), L"Trim when there are tabs at the end.");
+    VERIFY_ARE_EQUAL(L"Foo Bar", TrimPaste(L"Foo Bar\t\n"), L"Trim when there are tabs at the start of the whitespace at the end.");
+    VERIFY_ARE_EQUAL(L"Foo\tBar", TrimPaste(L"Foo\tBar\t\n"), L"Trim when there are tabs in the middle of the string, and in the whitespace at the end.");
+    VERIFY_ARE_EQUAL(L"Foo\tBar", TrimPaste(L"Foo\tBar\n\t"), L"Trim when there are tabs in the middle of the string, and in the whitespace at the end.");
+    VERIFY_ARE_EQUAL(L"Foo\tBar", TrimPaste(L"Foo\tBar\t\n\t"), L"Trim when there are tabs in the middle of the string, and in the whitespace at the end.");
+}
+void UtilsTests::TestDontTrimTrailingWhitespace()
+{
+    // Continue on failures
+    const WEX::TestExecution::DisableVerifyExceptions disableExceptionsScope;
+
+    VERIFY_ARE_EQUAL(L"Foo\tBar", TrimPaste(L"Foo\tBar"));
+
+    // Tests for GH #12387
+    VERIFY_ARE_EQUAL(L"Foo\nBar\n", TrimPaste(L"Foo\nBar\n"));
+    VERIFY_ARE_EQUAL(L"Foo  Baz\nBar\n", TrimPaste(L"Foo  Baz\nBar\n"));
+    VERIFY_ARE_EQUAL(L"Foo\tBaz\nBar\n", TrimPaste(L"Foo\tBaz\nBar\n"), L"Don't trim when there's a trailing newline, and tabs in the middle");
+    VERIFY_ARE_EQUAL(L"Foo\tBaz\nBar\t\n", TrimPaste(L"Foo\tBaz\nBar\t\n"), L"Don't trim when there's a trailing newline, and tabs in the middle");
+
+    // We need to both
+    // * trim when there's a tab followed by only whitespace
+    // * not trim then there's a tab in the middle, and the string ends in whitespace
+}

src/types/utils.cpp

@@ -772,3 +772,28 @@ std::tuple<std::wstring, std::wstring> Utils::MangleStartingDirectoryForWSL(std:
                                     std::wstring{ startingDirectory }
     };
 }
+
+std::wstring_view Utils::TrimPaste(std::wstring_view textView) noexcept
+{
+    const auto lastNonSpace = textView.find_last_not_of(L"\t\n\v\f\r ");
+    const auto firstNewline = textView.find_first_of(L"\n\v\f\r");
+
+    const bool isOnlyWhitespace = lastNonSpace == textView.npos;
+    const bool isMultiline = firstNewline < lastNonSpace;
+
+    if (isOnlyWhitespace)
+    {
+        // Text is all white space, nothing to paste
+        return L"";
+    }
+
+    if (isMultiline)
+    {
+        // In this case, the user totally wanted to paste multiple lines of text,
+        // and that likely includes the trailing newline.
+        // DON'T trim it in this case.
+        return textView;
+    }
+
+    return textView.substr(0, lastNonSpace + 1);
+}