Preserve wxRichTextCtrl append style stack

AppendText() moves the insertion point to the end before writing, which
refreshes the default style from the caret position. If a Begin*() style
is active, this could replace the explicit append style with the style at
the end of the existing text.

Save and restore the active default style when the rich text style stack
is non-empty, and cover appending text under a different text colour.

Fixes #10936.

Closes #27037.

src/richtext/richtextctrl.cpp

@@ -3381,8 +3381,14 @@ void wxRichTextCtrl::DoWriteText(const wxString& value, int flags)
 
 void wxRichTextCtrl::AppendText(const wxString& text)
 {
+    const bool hasStyleStack = GetBuffer().GetStyleStackSize() > 0;
+    const wxRichTextAttr defaultStyle = GetDefaultStyleEx();
+
     SetInsertionPointEnd();
 
+    if ( hasStyleStack )
+        SetDefaultStyle(defaultStyle);
+
     WriteText(text);
 }
 

tests/controls/richtextctrltest.cpp

@@ -699,6 +699,27 @@ TEST_CASE_METHOD(RichTextCtrlTestCase, "RichTextCtrl::TextColour",
     CHECK(colour.GetTextColour() == m_rich->GetBasicStyle().GetTextColour());
 }
 
+TEST_CASE_METHOD(RichTextCtrlTestCase, "RichTextCtrl::AppendTextStyle",
+                 "[richtextctrl]")
+{
+    m_rich->BeginTextColour(*wxRED);
+    m_rich->WriteText("red");
+    m_rich->EndTextColour();
+
+    m_rich->BeginTextColour(*wxBLUE);
+    m_rich->AppendText("blue");
+    m_rich->EndTextColour();
+
+    wxTextAttr colour;
+    m_rich->GetStyle(1, colour);
+
+    CHECK(colour.GetTextColour() == *wxRED);
+
+    m_rich->GetStyle(5, colour);
+
+    CHECK(colour.GetTextColour() == *wxBLUE);
+}
+
 TEST_CASE_METHOD(RichTextCtrlTestCase, "RichTextCtrl::NumberedBullet",
                  "[richtextctrl]")
 {