7012: top margin disappears upon resize in focus mode (#8140)

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## Summary of the Pull Request
In the focus mode the top border disappears upon resize. While this behavior is expected in the maximized / full screen mode, it should not happen in the focus mode.
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## References

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## PR Checklist
* [x] Closes #7012
* [x] CLA signed
* [ ] Tests added/passed - nope, only manual testing
* [ ] Documentation updated - irrelevant
* [ ] Schema updated - irrelevant
* [ ] 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

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## Detailed Description of the Pull Request / Additional comments
_GetTopBorderHeight method returns 0 when maximized or no title bar is visible. However the existence of top border has nothing to do with whether the title bar is visible. We want to leave the border as long as the window is not in some form of maximizing (maximized / full screen)
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## Validation Steps Performed
* Manual - dragging, resizing, maximizing both in focus and non focus modes + full screen testing

(cherry picked from commit 990628a)

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -263,9 +263,9 @@ void NonClientIslandWindow::SetTitlebarContent(winrt::Windows::UI::Xaml::UIEleme
 // - the height of the border above the title bar or 0 if it's disabled
 int NonClientIslandWindow::_GetTopBorderHeight() const noexcept
 {
-    // No border when maximized, or when the titlebar is invisible (by being in
-    // fullscreen or focus mode).
-    if (_isMaximized || (!_IsTitlebarVisible()))
+    // No border when maximized or fullscreen.
+    // Yet we still need it in the focus mode to allow dragging (GH#7012)
+    if (_isMaximized || _fullscreen)
     {
         return 0;
     }