Closes https://github.com/zed-industries/zed/issues/46926
## Description:
- Fixes the missing right border on pinned tabs when
`show_pinned_tabs_in_separate_row` is enabled.
The two-row tab bar layout was missing the border element that visually
separates the pinned tabs row. This border was present in the single-row
layout but was not added when implementing the two-row layout.
## Steps to reproduce
1. Enable `"tab_bar": { "show_pinned_tabs_in_separate_row": true }` in
settings
2. Open multiple files
3. Pin at least one tab (right-click → Pin Tab)
4. Notice pinned tabs are missing right-hand side border
**Before (bug):**
<img width="1060" height="186" alt="image"
src="https://github.com/user-attachments/assets/7d18f24d-a51b-4f13-8410-a15fa92e1bb3"
/>
**After (fixed):**
<img width="863" height="69" alt="image"
src="https://github.com/user-attachments/assets/3354e809-b298-49a1-b16a-871caef67176"
/>
## Test plan
- [x] Follow reproduction steps above and verify the border now appears
- [x] Verify border appears immediately (not just after drag-and-drop)
- [x] Verify single-row layout still works correctly when setting is
disabled
- [x] Added automated test `test_separate_pinned_row_has_right_border`
### Additional fix: Drag-and-drop to pinned tabs bar
During implementation, discovered that we couldn't drag tabs to the end
of the pinned tabs bar. This PR also adds:
- A drop target at the end of the pinned tabs row
- Proper pinning behavior when dropping unpinned tabs to the pinned area
- Keeps the dragged tab active after drop
Release Notes:
- Fixed the missing right border on pinned tabs when
`show_pinned_tabs_in_separate_row` is enabled.
- Fixed drop target at the end of the pinned tabs row
- Fixed pinning behavior when dropping unpinned tabs to the pinned area
- Fixed case when the dragged tab was not active after drop (when enable
`"tab_bar": { "show_pinned_tabs_in_separate_row": true }` in settings)
---------
Co-authored-by: Joseph T. Lyons <JosephTLyons@gmail.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
