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.test.tsx

@@ -266,12 +266,17 @@ describe("supersedes — who owns the prompt slot", () => {
 describe("TerminalView — where a dropped file lands", () => {
   /** Mount, let the async drag-drop registration settle, and give the pane a
    *  rect — jsdom has no layout, so every element is 0×0 and would be rejected
-   *  as a hidden pane. */
+   *  as a hidden pane.
+   *
+   *  The rect goes on the *pane wrapper*, which is what the hit test asks
+   *  about: it is what the user sees as the terminal (gutter included), and
+   *  the chrome painted over it — the Following toggle, the URL toast — are
+   *  its children rather than the xterm host's. */
   async function mountWithLayout() {
     const view = mountSession("bash");
     await act(async () => {});
-    const pane = view.container.querySelector(".xterm")?.parentElement;
-    if (!pane) throw new Error("terminal host element not found");
+    const pane = view.container.firstElementChild as HTMLElement | null;
+    if (!pane) throw new Error("terminal pane not found");
     pane.getBoundingClientRect = () =>
       ({
         left: 0,
@@ -287,6 +292,17 @@ describe("TerminalView — where a dropped file lands", () => {
     return view;
   }
 
+  /** jsdom has no `elementFromPoint`, so the z-order branch is unreachable
+   *  unless a test supplies one — which is exactly how a gate that refused
+   *  every drop under the Following toggle shipped through this file green. */
+  function stubElementFromPoint(top: Element | null) {
+    Object.defineProperty(document, "elementFromPoint", {
+      configurable: true,
+      writable: true,
+      value: () => top,
+    });
+  }
+
   async function drop(x: number, y: number) {
     if (!dragDrop.handler) throw new Error("no drag-drop listener registered");
     await act(async () => {
@@ -333,6 +349,47 @@ describe("TerminalView — where a dropped file lands", () => {
     await drop(400, 300);
     expect(vi.mocked(uploadHostFileToTerminal)).toHaveBeenCalledTimes(1);
   });
+
+  it("uploads a file dropped onto the always-present Following toggle", async () => {
+    // The regression this file could not see. The toggle is `absolute top-2
+    // right-4 z-50` and is rendered unconditionally, so `elementFromPoint`
+    // returns *it* for the terminal's top-right corner — and a gate asking
+    // "is what is painted here inside the xterm host?" answered no, forever,
+    // with no message and no log line. jsdom never ran that branch.
+    const view = await mountWithLayout();
+    const toggle = view.getByTitle(/Auto-scroll/i);
+    stubElementFromPoint(toggle);
+
+    await drop(780, 10);
+
+    expect(vi.mocked(uploadHostFileToTerminal)).toHaveBeenCalledWith(
+      "s1",
+      "/host/dropped.txt",
+    );
+    delete (document as Partial<Document>).elementFromPoint;
+  });
+
+  it("refuses — and says so — when a dialog is painted over the drop point", async () => {
+    await mountWithLayout();
+    const backdrop = document.createElement("div");
+    backdrop.setAttribute("data-blocks-drop", "true");
+    const panel = document.createElement("div");
+    panel.setAttribute("aria-modal", "true");
+    backdrop.appendChild(panel);
+    document.body.appendChild(backdrop);
+    stubElementFromPoint(backdrop);
+
+    await drop(400, 300);
+
+    expect(vi.mocked(uploadHostFileToTerminal)).not.toHaveBeenCalled();
+    // A refused drop is otherwise indistinguishable from a broken one.
+    expect(
+      useAppState.getState().toasts.some((t) => t.message === "File drop ignored"),
+    ).toBe(true);
+
+    backdrop.remove();
+    delete (document as Partial<Document>).elementFromPoint;
+  });
 });
 
 describe("TerminalView — reaching the URL prompt without a mouse", () => {