Fix crash when closing scratchpad pane

src/cascadia/TerminalApp/Pane.cpp

@@ -997,6 +997,16 @@ void Pane::_ContentLostFocusHandler(const winrt::Windows::Foundation::IInspectab
 // - <none>
 void Pane::Close()
 {
+    // GH#20187: Make Close() idempotent. The actual removal of this pane
+    // from the parent's tree is deferred until the close animation in
+    // _CloseChildRoutine completes (~200ms). Without this guard, a
+    // second close request that reaches this same pane during the
+    // animation window would re-raise Closed and kick off a redundant
+    // close animation on the parent.
+    if (!_content)
+    {
+        return;
+    }
     _setPaneContent(nullptr);
     // Fire our Closed event to tell our parent that we should be removed.
     Closed.raise(nullptr, nullptr);

src/cascadia/TerminalApp/Pane.h

@@ -93,6 +93,12 @@ class Pane : public std::enable_shared_from_this<Pane>
     winrt::Windows::UI::Xaml::Controls::Grid GetRootElement();
     winrt::TerminalApp::IPaneContent GetContent() const noexcept { return _IsLeaf() ? _content : nullptr; }
 
+    // GH#20187: True if this is a leaf pane whose content has already been
+    // torn down by Pane::Close() but which has not yet been removed from
+    // its parent (the close animation in _CloseChildRoutine is still
+    // running). Such a "corpse" pane is not safe to operate on.
+    bool IsClosed() const noexcept { return _IsLeaf() && _content == nullptr; }
+
     bool WasLastFocused() const noexcept;
     void UpdateVisuals();
     void ClearActive();

src/cascadia/TerminalApp/TabManagement.cpp

@@ -814,6 +814,23 @@ namespace winrt::TerminalApp::implementation
 
             if (const auto pane{ activeTab->GetActivePane() })
             {
+                // GH#20187: When the user closes a pane with a keyboard
+                // shortcut, focus moves off the closed pane while keys
+                // are still depressed. The corresponding KeyUp events
+                // arrive at TabRowControl, whose KeyUp is wired to
+                // TerminalPage::_KeyDownHandler (see TerminalPage.xaml).
+                // _KeyDownHandler can't tell it was invoked from KeyUp
+                // and dispatches the close action a second time. By
+                // then the parent's close animation in
+                // _CloseChildRoutine is still running, so
+                // Tab::_activePane still points at the now-closed leaf
+                // ("corpse"). Detect that here and bail before we
+                // dereference its null content in GetTerminalArgsForPane.
+                if (pane->IsClosed())
+                {
+                    co_return;
+                }
+
                 const auto weak = get_weak();
 
                 // Check if we should warn before closing a single pane