if tab clicked do imgui
for (unsigned i = 0; i < documents_.size(); i++) {
auto* doc = &documents_[i];
auto flag = doc->IsDirty() ? ImGuiTabItemFlags_UnsavedDocument : 0;
if (pending_selection_uid_ == doc->uid_) {
pending_selection_uid_ = 0;
flag |= ImGuiTabItemFlags_SetSelected;
}
if (ImGui::BeginTabItem(doc->Title().c_str(), &doc->opened_, flag)){
active_document_uid_ = doc->uid_;
ImGui::EndTabItem();
}
}