ImGuiFullscreen: Default input source to keyboard if none

src/util/imgui_fullscreen.cpp

@@ -675,8 +675,12 @@ ImGuiFullscreen::FocusResetType ImGuiFullscreen::GetQueuedFocusResetType()
 void ImGuiFullscreen::ForceKeyNavEnabled()
 {
   ImGuiContext& g = *ImGui::GetCurrentContext();
-  g.ActiveIdSource = (g.ActiveIdSource == ImGuiInputSource_Mouse) ? ImGuiInputSource_Keyboard : g.ActiveIdSource;
-  g.NavInputSource = (g.NavInputSource == ImGuiInputSource_Mouse) ? ImGuiInputSource_Keyboard : g.ActiveIdSource;
+  g.ActiveIdSource = (g.ActiveIdSource == ImGuiInputSource_Mouse || g.ActiveIdSource == ImGuiInputSource_None) ?
+                       ImGuiInputSource_Keyboard :
+                       g.ActiveIdSource;
+  g.NavInputSource = (g.NavInputSource == ImGuiInputSource_Mouse || g.NavInputSource == ImGuiInputSource_None) ?
+                       ImGuiInputSource_Keyboard :
+                       g.ActiveIdSource;
   g.NavDisableHighlight = false;
   g.NavDisableMouseHover = true;
 }