DebugConsole: restore non-history text on down key

When user starts typing something in the console, then navigates the
history with up but comes back to normal input by pressing down, restore
the text that was initially typed before navigating.

src/gui/DebugConsole.qml

@@ -19,6 +19,7 @@ HDrawer {
     property alias his: debugConsole.history
     property int historyEntry: -1
     property int maxHistoryLength: 4096
+    property var textBeforeHistoryNavigation: null  // null or string
 
     property string help: qsTr(
         `Javascript debugging console
@@ -131,9 +132,20 @@ HDrawer {
         }
     }
 
-    onHistoryEntryChanged:
-        inputArea.text =
-            historyEntry === -1 ? "" : history.slice(-historyEntry - 1)[0]
+    onHistoryEntryChanged: {
+        if (historyEntry === -1) {
+            inputArea.clear()
+            inputArea.append(textBeforeHistoryNavigation)
+            textBeforeHistoryNavigation = null
+            return
+        }
+
+        if (textBeforeHistoryNavigation === null)
+            textBeforeHistoryNavigation = inputArea.text
+
+        inputArea.clear()
+        inputArea.append(history.slice(-historyEntry - 1)[0])
+    }
 
     HShortcut {
         sequences: settings.keys.toggleDebugConsole