Docking: Fix unhiding tab bar regression. (#5325, #5181)

Broken by 90386780

imgui.cpp

@@ -6128,8 +6128,10 @@ void ImGui::RenderWindowDecorations(ImGuiWindow* window, const ImRect& title_bar
             float unhide_sz_hit = ImFloor(g.FontSize * 0.55f);
             ImVec2 p = node->Pos;
             ImRect r(p, p + ImVec2(unhide_sz_hit, unhide_sz_hit));
+            ImGuiID unhide_id = window->GetID("#UNHIDE");
+            KeepAliveID(unhide_id);
             bool hovered, held;
-            if (ButtonBehavior(r, window->GetID("#UNHIDE"), &hovered, &held, ImGuiButtonFlags_FlattenChildren))
+            if (ButtonBehavior(r, unhide_id, &hovered, &held, ImGuiButtonFlags_FlattenChildren))
                 node->WantHiddenTabBarToggle = true;
             else if (held && IsMouseDragging(0))
                 StartMouseMovingWindowOrNode(window, node, true);