Desktop: draw only the result rows on screen, and throttle progress events

[fstubner]

Gap 3 from the audit.

Measured

Production build, headless Chrome, a scan result of N rows, two runs each:

Rows	Arrow key (median)	Scroll step	Switching back to the tab
1,000	34 ms → 33 ms	34 → 33 ms	0.21 s → 34 ms
4,096	107–117 ms → 33 ms	59–65 → 33 ms	0.9 s → 33 ms
20,000	620–660 ms → 50–66 ms	313–321 → 33 ms	4.7–5.5 s → 100 ms

33 ms is the measurement floor: two animation frames. 4,096 is a full port scan; 20,000 rows is a long packet capture.

What changed

• Windowed rows (rowWindow.ts). Above 200 rows the table renders the rows in view plus 20 either side, between two spacer rows that keep the scrollbar the full size. Below 200 everything renders as before.
• Keyboard focus scrolls the focused row into view itself, since a row that is not rendered cannot scrollIntoView. aria-rowcount and aria-rowindex keep the true row count for assistive tech.
• No re-render on every scroll. The scroll handler set the overflow state each time, even with no edge changed, which re-rendered every row.
• Progress events throttled in the Tauri backend: at most one per operation per 100 ms, plus the first, the last and each phase change. Before, a full port scan sent 4,096 IPC events, each a React update.

Checked

• On 4,096 rows: the middle of the result renders with no gaps, and End lands on the last row, on screen, with aria-activedescendant pointing at it (screenshots in the session).
• Unit tests for the window and scroll maths, and for the throttle (a 4,096-port scan inside one interval sends 2 events; phases and operations are throttled separately).
• Desktop: typecheck, eslint, 233 unit tests, vite build, cargo clippy -p netscli-gui -D warnings, its 25 Rust tests.

The spacer CSS is in its own file because results.css is at its size cap.

[github-actions]

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