Fix inactive pane dimming on dark themes and make intensity configurable

[Akeuuh]

Description

Fixes inactive-pane dimming on dark themes and makes the dimming intensity user-configurable.

Bug fix (GH#5401): the "Dim inactive panes" overlay blended toward the theme foreground color. On dark themes the foreground is light, so dimming actually brightened inactive panes. The overlay now fades toward the window background, so inactive panes recede on both light and dark themes. (WarpTheme::inactive_pane_overlay_with_opacity; the legacy inactive_pane_overlay used for modal/header scrims is unchanged.)

New setting: appearance.panes.inactive_pane_dimming_percentage (u8, 0–100, default 10), surfaced as a slider under Settings → Appearance → Panes, shown only when "Dim inactive panes" is enabled. The pane re-renders on PaneSettingsChangedEvent::InactivePaneDimmingPercentage; synced to cloud like the existing toggle.

Files: app/src/settings/pane.rs, app/src/pane_group/pane/view/mod.rs, app/src/settings_view/appearance_page.rs, crates/warp_core/src/ui/theme/color.rs.

Linked Issue

Closes #5401

Supersedes #11472 (filed by me, then flagged as a duplicate of #5401 by Oz).

Note: feature/bug request — opening as draft pending a ready-to-spec / ready-to-implement label (and a spec under specs/ if the team wants one) per CONTRIBUTING.md.

Testing

• Manually tested locally with ./script/run (macOS OSS build): on a dark theme, enabling "Dim inactive panes" now darkens the inactive split instead of brightening it; the slider changes intensity live; default (10) is a subtle dim.
• cargo fmt -- --check ✅
• cargo clippy --workspace --exclude warp_completer --all-targets --tests -- -D warnings ✅ (+ -p warp_completer ✅)
• cargo test --doc ✅
• Touched-code unit tests pass; the only local nextest failures were environment-dependent (SSH integration tests needing a backend, and a non-hermetic settings-migration test that reads the real ~/.warp/settings.toml), unrelated to this change.

Screenshots / Videos

Enregistrement.de.l.ecran.2026-05-21.a.14.40.03.mov

Agent Mode

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

CHANGELOG-BUG-FIX: "Dim inactive panes" now dims (rather than brightens) inactive panes on dark themes.
CHANGELOG-IMPROVEMENT: Inactive pane dimming intensity is now configurable in Settings → Appearance → Panes.

[oz-for-oss]

@Akeuuh

This PR is not linked to an issue that is marked with ready-to-implement.

Issue-state enforcement details:

• Associated same-repo issues checked: Add setting to adjust Pane dimming radiance, since "Dim inactive panes" doesn't actually dim in dark themes, it lightens #5401

• Required readiness label: ready-to-implement

Readiness check:

• Add setting to adjust Pane dimming radiance, since "Dim inactive panes" doesn't actually dim in dark themes, it lightens #5401: missing ready-to-implement; readiness labels present: none

To continue, link this PR to a same-repo issue such as Closes #123 in the PR description, and make sure that issue has ready-to-implement.

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[oz-for-oss]

@Akeuuh

This PR is not linked to an issue that is marked with ready-to-implement.

Issue-state enforcement details:

• Associated same-repo issues checked: #5401

• Required readiness label: ready-to-implement

Readiness check:

• #5401: missing ready-to-implement; readiness labels present: none

To continue, link this PR to a same-repo issue such as Closes #123 in the PR description, and make sure that issue has ready-to-implement.

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