Stop the terminal and Files panes refusing drops onto their own chrome

The z-order gate added last round asked `el.contains(elementFromPoint(x, y))`
— "is the thing painted here mine?" — and was handed `TerminalView`'s inner
xterm host while every overlay in that pane is a *sibling* of it. So any point
under the pane's own chrome answered "not mine" and the drop was refused, with
no message and no log line. The "▼ Following / ▽ Paused" toggle is rendered
unconditionally at `absolute top-2 right-4`, and `ToastHost` is `fixed
bottom-4 right-4` 24rem wide with error cards that never time out: two corners
of the terminal, and one of the Files pane, that could not accept a file for
as long as the app was running.

It shipped green because jsdom has no `elementFromPoint`, so not one of the 81
drop tests entered that branch. The tests here install one.

The question the gate asks is now "is a *blocking overlay* painted here?".
Chrome the pane paints over itself is not one; a dialog backdrop is, and
`ui/Modal` marks its own backdrop so the element `elementFromPoint` actually
returns is the one carrying the marker. `classifyDrop` also separates "aimed at
me and swallowed" from "not my drop", so the first gets a toast and a log line
and the second stays silent.

Three defects around it:

- **A dialog now refuses only the points it covers.** `dropIsBlocked` is
  document-wide and `ui/Modal` portals to `document.body`, so any open dialog
  refused every drop in the window. The deeper half of that is that a dialog
  opened in project A really was still on screen after a tab switch — the pane
  hides itself with a `hidden` class, which a portal does not inherit — so
  `PaneVisibility` lets `App` tell a `Modal` its pane stepped aside, and a
  hidden one paints nothing, traps no focus, answers no Escape and blocks no
  drop while staying mounted with its state intact.

- **`devicePixelRatio` is applied on Windows only.** Only wry's WebView2
  backend hands over physical pixels; the macOS and GTK ones deliver logical
  points and `tauri-runtime-wry` does not rescale them. Halving those was
  survivable while the test was a bare rect and is a refused drop once z-order
  joins in. Read from the wry/tauri sources, not verified on a HiDPI Mac or
  GTK box.

- **`isFileExistsError` can no longer be forged by a filename.** It matched
  `fileexists` anywhere in a normalised error, so uploading a host file called
  `file-exists.txt` turned *any* failure into a collision — and Replace
  re-invoked the upload with `overwrite: true`. The marker now has to stand
  alone in the backend's canonical form, or be a whole discriminant value.

- **A refused compaction or cache-clear keeps its dialog.** `reclaim` reports
  refusals inside `Ok`, so "did it throw" read one as success: the dialog
  closed, the tick list was dropped, and the explanation appeared in the
  outcome panel several screens above the row that was clicked. The dialog now
  stays put and renders the backend's own sentence verbatim.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBq2rGum6GX7xXgsas1fDc

app/src/components/terminal/TerminalView.tsx

@@ -21,7 +21,7 @@ import {
   parseUrlRelayOsc,
   sanitizeRelayUrl,
 } from "../../lib/urlRelay";
-import { isDropTarget } from "../../lib/dropTarget";
+import { classifyDrop } from "../../lib/dropTarget";
 import UrlToast, {
   URL_TOAST_PRIMARY_SELECTOR,
   URL_TOAST_SELECTOR,
@@ -237,14 +237,22 @@ export default function TerminalView({ sessionId, active }: Props) {
   //
   // The listener is window-wide, so every pane decides for itself whether a
   // drop was meant for it. `isDropTarget` is that decision, shared with the
-  // Files pane: the physical-pixel position ÷ `devicePixelRatio` against this
-  // pane's rect (a hidden pane is `display:none`, so its zero-size rect is what
-  // stops two panes both claiming the drop), plus z-order — which a rect alone
-  // cannot see. An open `Modal` is a `fixed inset-0` portal painted *over* the
-  // window and the pane underneath still has its rect, so the geometric test
-  // that used to live here uploaded files into the directory a dialog was
-  // covering. Same for the shutdown overlay, which is on screen precisely while
-  // nothing should be accepting work at all.
+  // Files pane: the payload position against this pane's rect (a hidden pane is
+  // `display:none`, so its zero-size rect is what stops two panes both claiming
+  // the drop), plus z-order — which a rect alone cannot see. An open `Modal` is
+  // a `fixed inset-0` portal painted *over* the window and the pane underneath
+  // still has its rect, so the geometric test that used to live here uploaded
+  // files into the directory a dialog was covering. Same for the shutdown
+  // overlay, which is on screen precisely while nothing should be accepting
+  // work at all.
+  //
+  // The rect asked about is the **pane wrapper**, not the xterm host inside it:
+  // the pane is what the user sees as "the terminal", gutter included, and the
+  // chrome painted over it (the Following toggle, the URL toast) is a sibling
+  // of the host rather than a child. `classifyDrop` answers "is a *blocking
+  // overlay* here?" rather than "is this element mine?" for the same reason —
+  // asking the second question turned every pixel under that chrome into a
+  // permanent dead zone.
   useEffect(() => {
     let unlisten: (() => void) | undefined;
     let cancelled = false;
@@ -257,7 +265,29 @@ export default function TerminalView({ sessionId, active }: Props) {
     (async () => {
       const un = await getCurrentWebview().onDragDropEvent(async (event) => {
         if (event.payload.type !== "drop") return;
-        if (!isDropTarget(containerRef.current, event.payload.position)) return;
+        const verdict = classifyDrop(
+          terminalContainerRef.current,
+          event.payload.position,
+        );
+        // A refused drop is invisible — the file simply does not arrive — so
+        // the one case where the user aimed at us and we said no gets both a
+        // log line and something on screen. The toast, not `imagePasteMsg`:
+        // whatever refused this is painted over the terminal, and `ToastHost`
+        // sits above it.
+        if (verdict === "blocked") {
+          console.warn(
+            "[drop] refused: an overlay is covering the drop point",
+            event.payload.position,
+          );
+          useAppState.getState().pushToast({
+            kind: "info",
+            message: "File drop ignored",
+            detail:
+              "A dialog or full-window overlay is covering that point. Close it and drop the file again.",
+          });
+          return;
+        }
+        if (verdict !== "accept") return;
 
         const paths = event.payload.paths ?? [];
         if (paths.length === 0) return;