ui: Fix content shift when selecting last tab (#31266)

Follow-up to #29061

This PR ensures that the last tab does not flicker when either
selecting. It also fixes an issue where the layout would shift in the
new last tab when closing the last tab.


https://github.com/user-attachments/assets/529a2a92-f25c-4ced-a992-fb6b2d3b5f61

This happened because in #29061, the left padding was removed due to
issues with borders. However, the padding is relevant for the content to
not shift (we are basically doing border-box sizing manually here).
Instead, we need to remove the padding on the right side, as there is
already a border present on the right side and this padding would make
the last tab slightly larger than all other tabs.


https://github.com/user-attachments/assets/c3a10b3c-4a1d-4160-9b68-7538207bb46e


Release Notes:

- Removed a small flicker when selecting or closing the last tab in a
pane.

crates/ui/src/components/tab.rs

@@ -154,7 +154,7 @@ impl RenderOnce for Tab {
                     if self.selected {
                         this.border_l_1().border_r_1().pb_px()
                     } else {
-                        this.pr_px().border_b_1().border_r_1()
+                        this.pl_px().border_b_1().border_r_1()
                     }
                 }
                 TabPosition::Middle(Ordering::Equal) => this.border_l_1().border_r_1().pb_px(),