Agentic UI: fix resizer for chat input

[nightnei]

Related issues

• Fixes STU-2062

How AI was used in this PR

AI assisted

Proposed Changes

1. Electron -> Beta features -> Enable Agentic UI
2. Mouse over at the top chat input border and assert that you see a draggable line, with styles aligned with teh left sidebar resizer:
   
3. Click anywhere in the UI to focus teh window and then click tab button a few times to reach the chat input resizer

BEFORE	AFTER

4. Click up/down buttons on your keyboard and assert that resizing works from the keyboard when the resizer is focused

[wpmobilebot]

📊 Performance Test Results

Comparing b01b688 vs trunk

app-size

Metric	trunk	b01b688	Diff	Change
App Size (Mac)	1362.03 MB	1361.91 MB	0.12 MB	⚪ 0.0%

site-editor

Metric	trunk	b01b688	Diff	Change
load	1077 ms	1066 ms	11 ms	⚪ 0.0%

site-startup

Metric	trunk	b01b688	Diff	Change
siteCreation	6969 ms	6982 ms	+13 ms	⚪ 0.0%
siteStartup	2350 ms	2350 ms	0 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[bcotrim]

Suggested change

	.shell:focus-within:not(:has(.resizeHandle:focus-visible)) {

Adding this prevents the double line.
What do you think?

Light	Dark

[sejas]

LGTM. The selected top border looks better. I wonder if we can improve it a bit more around the borders but I think it's a minor thing and now it's much better. Thanks!

Before	After

[shaunandrews]

the weird focus border was a genuine issue; but I avoided having a separate drag handle for the top border, mostly to avoid competing with the focus outline. In this case I felt it find to rely on the cursor changing on hover as a way to discover.