fix hidden "Add signal" button

src/ui/flowgraphitem.cpp

@@ -614,8 +614,11 @@ void FlowGraphItem::draw(FlowGraph *fg, const ImVec2 &size) {
         ImGui::OpenPopup("New block");
     }
 
-    ImGui::SetCursorPosX(left);
-    ImGui::SetCursorPosY(top + size.y - 20);
+    // Create a new ImGui window for an overlay over the NodeEditor , where we can place our buttons
+    // if we don't put the buttons in this overlay, they will be overdrawn by the editor
+    ImGui::SetNextWindowPos(ImVec2(0, top + size.y - 30), ImGuiCond_Always);
+    ImGui::Begin("Button Overlay", nullptr, ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoScrollbar |ImGuiWindowFlags_NoBackground);
+
     if (ImGui::Button("Add signal")) {
         ImGui::OpenPopup("addSignalPopup");
     }
@@ -628,6 +631,8 @@ void FlowGraphItem::draw(FlowGraph *fg, const ImVec2 &size) {
     drawAddSourceDialog(fg);
     drawNewBlockDialog(fg);
 
+    ImGui::End(); // overlay
+
     if (horizontalSplit) {
         const float w = size.x * ratio;
         ImGuiUtils::drawBlockControlsPanel(m_editPane, { left + size.x - w + halfSplitterWidth, top }, { w - halfSplitterWidth, size.y }, true);