Fix preview legend window size (#3363)

Fix issue that the legend window expands to all available horizontal space if scrollbar is displayed

(cherry picked from commit a8ade15)

src/slic3r/GUI/GCodeViewer.cpp

@@ -4504,16 +4504,17 @@ void GCodeViewer::render_legend(float &legend_height, int canvas_width, int canv
         return ret;
     };
 
-    auto calculate_offsets = [max_width, window_padding](const std::vector<std::pair<std::string, std::vector<::string>>>& title_columns, float extra_size = 0.0f) {
+    auto calculate_offsets = [&imgui, max_width, window_padding](const std::vector<std::pair<std::string, std::vector<::string>>>& title_columns, float extra_size = 0.0f) {
             const ImGuiStyle& style = ImGui::GetStyle();
             std::vector<float> offsets;
             offsets.push_back(max_width(title_columns[0].second, title_columns[0].first, extra_size) + 3.0f * style.ItemSpacing.x);
             for (size_t i = 1; i < title_columns.size() - 1; i++)
                 offsets.push_back(offsets.back() + max_width(title_columns[i].second, title_columns[i].first) + style.ItemSpacing.x);
             if (title_columns.back().first == _u8L("Display")) {
-                const auto preferred_offset = ImGui::GetWindowWidth() - ImGui::CalcTextSize(_u8L("Display").c_str()).x - ImGui::GetFrameHeight() / 2 - 2 * window_padding;
+                const auto preferred_offset = ImGui::GetWindowWidth() - ImGui::CalcTextSize(_u8L("Display").c_str()).x - ImGui::GetFrameHeight() / 2 - 2 * window_padding - ImGui::GetStyle().ScrollbarSize;
                 if (preferred_offset > offsets.back()) {
                     offsets.back() = preferred_offset;
+                    imgui.set_requires_extra_frame();
                 }
             }