Tab width top border

Following the tabs example, If a fixed width is used with less tabs
```go
doc.WriteString(windowStyle.Width(100).Render(m.TabContent[m.activeTab]))
```


The top border is missing. How to fix this?

Start with the tabs example and reproduce the issue using a fixed width with fewer tabs, as shown in the report. Inspect how Width(100) and Render interact with the tab content and top border. Done means the top border renders correctly in this case, with a test or example confirming the behavior.

Written by the indexing model from the issue text.