Make Agent Manager session tabs compact

[marius-kilocode]

Summary

• Make Agent Manager session tabs compact with deterministic, Chrome-style sizing and keep the new tab control next to the last tab.
• Show only meaningful session status in session tabs, so idle tabs stay title-first and running tabs show a spinner.
• Keep close targets stable during repeated mouse closes by freezing widths until the pointer leaves, then reflowing evenly.

Why

Agent Manager encourages many parallel sessions, so a fixed-width tab strip quickly becomes hard to scan and easy to misclick. Matching Chrome's tab behavior gives users a familiar model: tabs shrink evenly as more are opened, the new-tab affordance stays near the tab group, and active or hovered tabs expose close controls without changing tab identity.

Design choices

• Deterministic CSS sizing: tab width is derived from tab count and available width, not runtime measurement, so startup and tab switching do not animate through intermediate sizes.
• Title-first session tabs: idle sessions do not need a default icon, because it competes with the title and can imply the tab is a worktree. Running sessions still show a spinner because that state is actionable.
• Stable close targets: removing tabs with the pointer over the strip freezes measured widths, preventing the close button from moving under the cursor while the user closes several tabs in a row. Adding tabs reflows immediately because there is no repeated-close risk.
• Inline new-tab control: the plus button lives next to the rightmost tab instead of at the far edge, keeping tab creation spatially tied to the tab group.

Closes #9954

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• packages/kilo-docs/public/img/screenshot-tests/kilo-vscode/visual-regression/agentmanager/tab-bar-multiple-tabs-chromium-linux.png
• packages/kilo-docs/public/img/screenshot-tests/kilo-vscode/visual-regression/agentmanager/tab-bar-single-tab-chromium-linux.png
• packages/kilo-docs/public/img/screenshot-tests/kilo-vscode/visual-regression/agentmanager/tab-bar-with-review-tab-chromium-linux.png

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_{Reviewed by gpt-5.5-2026-04-23 · 485,287 tokens}