feat: per-pane monospace font size

[wpoulin]

Description

Adds independent monospace font-size control for each terminal pane (issue #6739). Previously the font-size shortcuts adjusted every Warp window globally; now each pane can have its own size, so users can shrink the text in one pane to see more output while keeping a sibling pane large for readability.

User-visible behavior

The shortcuts depend on whether the UIZoom flag is enabled (UIZoom repurposes Cmd+= / Cmd+- / Cmd+0 for window zoom):

• Without UIZoom: Cmd+= / Cmd+- adjust the focused pane's font size (previously global). Cmd+0 resets the focused pane's override back to the global default.
• With UIZoom: Ctrl+Shift+> / Ctrl+Shift+< adjust the focused pane's font size (the = / - keys are taken by
  window zoom). Cmd+0 is bound to ResetZoom, and now also clears the focused pane's font-size override — same key resets both.

In both cases:

• The terminal grid, prompt, alt-screen, and the input editor where the user types all pick up the pane's effective font size — no glyph/cell mismatch and no leftover full-size input box. UI chrome (banners, menus, AI mode indicators) intentionally stays on the global so per-pane overlays don't change size as you adjust the terminal text.
• The Settings UI's monospace font size slider continues to write the global; panes without an override continue to follow it.

Implementation

• Per-pane override is stored as HashMap<PaneId, f32> on PaneGroupFocusState; emits a new FontSizeOverrideChanged { pane_id } event with PaneFocusHandle::is_affected filtering to the matching pane.
• The render path is threaded with the pane's effective size — SizeUpdateBuilder::build, BlockListElement,
  BlockGridElement, AltScreenElement, and the prompt render helpers — so cell metrics and glyph painting always change together.
• EditorView already had set_font_size; added clear_font_size and made set_font_size idempotent. Input syncs the editor's override on every FontSizeOverrideChanged and once at focus-handle wiring time so a freshly split pane that inherits an override picks it up before its first render.
• Override entries are dropped on permanent pane removal — close_pane (non-undo path), cleanup_closed_pane (undo grace expiry), remove_pane_for_move (pane leaves this group). The UndoClosedPanes restore path correctly preserves the override (we only forget on permanent removal).
• Override is in-memory only by design: PaneIds aren't stable across restarts, so persisting wouldn't be meaningful.

Linked Issue

Closes #6739.

• The linked issue is labeled ready-to-spec or ready-to-implement. (labeled Ready-to-Implement)
• Where appropriate, screenshots or a short video of the implementation are included below (especially for
  user-visible or UI changes).

Screenshots / Videos

Screen.Recording.2026-04-30.at.11.28.16.PM.mov

Split the window into two panes, then increase the font size in one pane and decrease it in the other — each pane scales independently. Resetting returns both panes to the default size.

Testing

Unit tests added (8 total, all passing):

• Cover override behavior, precedence over global, cleanup, no-ops, event targeting, and editor integration

Manual end-to-end testing in a local debug build (verified on both UIZoom on and off):

• Verified independent resizing across panes
• Reset behavior (Cmd+0) works in both modes
• Global font changes don’t affect overridden panes
• Works correctly in alt-screen apps (vim, htop)
• Overrides persist through undo-close, but not permanent close

Presubmit: ./script/presubmit — cargo fmt, cargo clippy --workspace --all-targets --tests -- -D warnings,
clang-format, and wgslfmt --check all clean.

Known limitation (out of scope, follow-up issue suggested): Per-pane override is dropped when a pane is dragged to another window — the destination group starts at the global default. Preserving across moves would need override state to live on pane content metadata; left for a separate PR.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

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[wpoulin]

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Overview

This PR adds in-memory per-pane monospace font-size overrides, routes the effective pane font size through terminal sizing/render paths, and syncs the embedded input editor with pane-level override changes.

Concerns

• No blocking correctness, security, error-handling, or performance concerns found in the inlined diff.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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