gui: Avoid pathological QT text/markdown behavior...

during text selection by only setting plaintext mime data.

src/qt/forms/debugwindow.ui

@@ -573,7 +573,7 @@
         </layout>
        </item>
        <item>
-        <widget class="QTextEdit" name="messagesWidget">
+        <widget class="PlainCopyTextEdit" name="messagesWidget">
          <property name="minimumSize">
           <size>
            <width>0</width>
@@ -1868,6 +1868,10 @@
     <slot>clear()</slot>
    </slots>
   </customwidget>
+  <customwidget>
+   <class>PlainCopyTextEdit</class>
+   <extends>QTextEdit</extends>
+  </customwidget>
  </customwidgets>
  <resources>
   <include location="../bitcoin.qrc"/>

src/qt/rpcconsole.h

@@ -15,6 +15,9 @@
 
 #include <QByteArray>
 #include <QCompleter>
+#include <QMimeData>
+#include <QTextDocumentFragment>
+#include <QTextEdit>
 #include <QThread>
 #include <QWidget>
 
@@ -191,4 +194,20 @@ private Q_SLOTS:
     void updateAlerts(const QString& warnings);
 };
 
+/**
+ * A version of QTextEdit that only populates plaintext mime data from a
+ * selection, this avoids some bad behavior in QT's HTML->Markdown conversion.
+ */
+class PlainCopyTextEdit : public QTextEdit {
+    Q_OBJECT
+public:
+    using QTextEdit::QTextEdit;
+protected:
+    QMimeData* createMimeDataFromSelection() const override {
+        auto md = new QMimeData();
+        md->setText(textCursor().selection().toPlainText());
+        return md;
+    }
+};
+
 #endif // BITCOIN_QT_RPCCONSOLE_H