Closes#2915. `.agent-detail-overlay` has had **no renderer** since
FN-8619 moved Agent Detail onto `FloatingWindow`, whose `modal` host
owns the scrim (`.floating-window-overlay--modal`). Four CSS sites plus
one inert mobile `@media` rule.
## Why this sat unfixed, and what unblocked it
I filed this earlier and deliberately did **not** delete the CSS,
because two *passing* guards pinned a selector list naming the class:
```
dashboard-overflow-containment.test.tsx:295
mobile-horizontal-pan-containment.test.ts:90
".modal-overlay:not(.confirm-dialog-overlay),\n .agent-detail-overlay,\n .agent-dialog-overlay,\n .workflow-output-modal-overlay"
```
Neither list mentions `.floating-window-overlay--modal`. If that list
were the mechanism keeping modal scrims inside mobile horizontal-pan
containment, then FN-8619 moved every migrated modal's scrim out from
under the guard and the dead entry was **masking a live defect** —
deleting it would have been the wrong move twice over.
I said at the time I couldn't settle it without rendering at a phone
breakpoint. **That was wrong — it is answerable by reading**, and I only
went back because the same mistaken conclusion cost me a day on the
Planning coverage in #2982.
**The containment lockdown is global**, on the mobile `html, body` block
in `styles.css`:
```css
@media (max-width: 768px) {
html, body {
overflow-x: hidden;
overscroll-behavior-x: none;
touch-action: pan-y;
}
```
`FloatingWindow.css` says so itself at its mobile breakpoint — *"Mobile
keeps the global `styles.css` pan-y lockdown so the dashboard cannot
drift"*. The per-overlay list only reasserts it for overlays that are
themselves scroll containers. Migrated modals are covered by the global
rule, so **no hole, and no masked defect**.
## Verified the guards still guard something
The risk in editing a pinned selector string is turning a real guard
into a string-equality formality:
| state | result |
|---|---|
| after this change | 15/15 pass |
| global lockdown broken (`touch-action` / `overscroll-behavior-x`
removed from `html, body`) | **2 failed / 13 passed** |
They fail on the mechanism, not the text.
## Scope note
Two of the four `styles.css` sites are **grouped selectors shared with
`.agent-dialog-overlay`, which is still live**
(`NewAgentDialog.tsx:415`). So this is a selector-list edit, not a block
deletion — easy to get wrong in a bulk sweep, which is why it is called
out here and in the FNXC note replacing the deleted rule.
The retired mobile rule set `padding: 0; align-items: stretch` on the
overlay. Not a lost feature: `.floating-window-overlay` is `position:
fixed; inset: 0` with no flex context, so those declarations had nothing
to act on — FloatingWindow positions the panel by geometry.
**Verified:** 120/120 across `agent-modals-mobile`,
`core-modals-mobile`, `AgentDetailView.core`, and both containment
guards; `tsc -p tsconfig.app.json` 0 errors; lint clean; FNXC gate exit
0.
Dead-CSS removal with no behaviour change, so no changeset. Main health
while I was here: engine **11475 passed / 0 failed**.
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
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