Correct for the size of the tabs when calculating our initial window size (#4825)

## Summary of the Pull Request

This fixes our calculation for the initial size of the window. WE weren't accounting for the height of the tabs, so the `initialRows` was consistently wrong.

## PR Checklist
* [x] Closes #2061
* [x] I work here
* [ ] Tests added/passed
* [n/a] Requires documentation to be updated

## Detailed Description of the Pull Request / Additional comments

For the tabs below the titlebar case, there's 6px (unscaled) of space that I cannot account for. I seriously have no idea where it's coming from. When we end up creating the first `TermControl` after startup, there's an inexplicable `6*scale` difference between the height of the `tabContent` and the `SwapChainPanel`'s size.

## Validation Steps Performed

Checked all six of the following cases:
* 1.0 DPI scaling, Tabs in Titlebar
* 1.25 DPI scaling, Tabs in Titlebar
* 1.0 DPI scaling, Tabs NOT in Titlebar, always show tabs
* 1.0 DPI scaling, Tabs NOT in Titlebar, DON'T always show tabs
* 1.25 DPI scaling, Tabs NOT in Titlebar, always show tabs
* 1.25 DPI scaling, Tabs NOT in Titlebar, DON'T always show tabs

src/cascadia/TerminalApp/AppLogic.cpp

@@ -400,11 +400,44 @@ namespace winrt::TerminalApp::implementation
         // Use the default profile to determine how big of a window we need.
         const auto [_, settings] = _settings->BuildSettings(nullptr);
 
-        // TODO MSFT:21150597 - If the global setting "Always show tab bar" is
+        auto proposedSize = TermControl::GetProposedDimensions(settings, dpi);
+
+        const float scale = static_cast<float>(dpi) / static_cast<float>(USER_DEFAULT_SCREEN_DPI);
+
+        // GH#2061 - If the global setting "Always show tab bar" is
         // set or if "Show tabs in title bar" is set, then we'll need to add
         // the height of the tab bar here.
+        if (_settings->GlobalSettings().GetShowTabsInTitlebar())
+        {
+            // If we're showing the tabs in the titlebar, we need to use a
+            // TitlebarContol here to calculate how much space to reserve.
+            //
+            // We'll create a fake TitlebarControl, and we'll propose an
+            // available size to it with Measure(). After Measure() is called,
+            // the TitlebarControl's DesiredSize will contain the _unscaled_
+            // size that the titlebar would like to use. We'll use that as part
+            // of the height calculation here.
+            auto titlebar = TitlebarControl{ static_cast<uint64_t>(0) };
+            titlebar.Measure({ SHRT_MAX, SHRT_MAX });
+            proposedSize.Y += (titlebar.DesiredSize().Height) * scale;
+        }
+        else if (_settings->GlobalSettings().GetAlwaysShowTabs())
+        {
+            // Otherwise, let's use a TabRowControl to calculate how much extra
+            // space we'll need.
+            //
+            // Similarly to above, we'll measure it with an arbitrarily large
+            // available space, to make sure we get all the space it wants.
+            auto tabControl = TabRowControl();
+            tabControl.Measure({ SHRT_MAX, SHRT_MAX });
 
-        return TermControl::GetProposedDimensions(settings, dpi);
+            // For whatever reason, there's about 6px of unaccounted-for space
+            // in the application. I couldn't tell you where these 6px are
+            // coming from, but they need to be included in this math.
+            proposedSize.Y += (tabControl.DesiredSize().Height + 6) * scale;
+        }
+
+        return proposedSize;
     }
 
     // Method Description:

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -492,6 +492,8 @@ SIZE NonClientIslandWindow::GetTotalNonClientExclusiveSize(UINT dpi) const noexc
 
     islandFrame.top = -topBorderVisibleHeight;
 
+    // If we have a titlebar, this is being called after we've initialized, and
+    // we can just ask that titlebar how big it wants to be.
     const auto titleBarHeight = _titlebar ? static_cast<LONG>(_titlebar.ActualHeight()) : 0;
 
     return {