claude-code - 💡(How to fix) Fix Plan side panel: content stops expanding at a fixed width — large wasted empty margins on wider panel

Description

When the right-hand Plan side panel is resized to a wider width, the content inside the panel does not expand to fill the available space. Instead, it stops at a fixed max-width and leaves large empty margins on both sides of the content column (clearly visible as wasted whitespace on the left and right of the text inside the Plan panel).

This makes wider panel widths effectively useless — there is no benefit to dragging the divider, since the readable content area stays the same and the user only gains empty space.

<img width="3392" height="2202" alt="Image" src="https://github.com/user-attachments/assets/38080dd3-02f3-48f9-8a9d-44401425adff" /> <img width="3392" height="2200" alt="Image" src="https://github.com/user-attachments/assets/2fd15419-60d8-474a-90a5-527a7b6645e5" />

Steps to reproduce

1. Open the Claude Code desktop app on macOS.
2. Open a session that has a Plan attached (any plan proposal flow works).
3. Open the right-hand Plan side panel.
4. Drag the panel divider to make the Plan panel significantly wider (e.g. ~⅓ to ~½ of the window).
5. Observe the content inside the Plan panel.

The text/markdown content stays at a fixed width in the middle of the panel; the rest is empty padding on the left and right.

Expected behavior

The content inside the Plan side panel should expand intelligently as the panel is widened:

• A reasonable max line-length for readability is fine as a soft cap, but the cap should be generous and the wasted whitespace should be minimal.
• Alternatively, content blocks like code snippets, tables, and the Implementation Plan card should grow to make use of the additional horizontal space, since they benefit from width more than prose paragraphs do.
• The user should perceive a clear benefit from widening the panel (more content visible per line, less vertical scrolling).

Actual behavior

The content column inside the Plan panel is capped at a fixed width well below the panel's actual width. Both sides of the content show large empty margins. Widening the panel only increases the empty margins, not the readable area.

Reference — competitor behavior (Antigravity 2.0 by Google)

Google's Antigravity 2.0 app handles this case well: when the side panel is widened, the content (markdown, code blocks, plan view) expands to use the additional space, with no wasted margins. The panel feels responsive and the extra width is actually useful.

Claude Code desktop should adopt the same behavior — the side panel should be a true resizable workspace, not a fixed-width column floating inside a wider container.

Environment

• App: Claude Code desktop app (macOS)
• OS: macOS (Darwin 25.5.0)
• Plan: Max
• Model in session: Opus 4.7 (1M context)
• Reproducibility: deterministic — happens any time the Plan side panel is widened beyond the internal fixed cap

Screenshots

Two screenshots to drag into the issue after opening it:

1. Claude Code desktop with the Plan side panel widened — large red overlays mark the wasted empty margins on both sides of the content.
2. Antigravity 2.0 with a comparable side panel — content expands to fill the panel correctly, with a green annotation: "Antigravity 2.0 works great and sidebar correctly expands its content".