It would be nice if there was an option provided to hide the tab bar when there's only one file open. This would match the behavior of most apps that uses Libadwaita. I have made myself a CSS snippet that does this, but I think it could be included as an option in Style Settings.
.mod-root .workspace-tab-header-container-inner:not(:has(> .workspace-tab-header:nth-child(2))) > .workspace-tab-header {
display: none;
}