fix(fork): clip the composer placeholder out of the prompt scrollport

.fork/customizations.yaml

@@ -938,8 +938,16 @@
     inflate scrollHeight on the editor, so upstream's max-h + overflow-y:auto
     there painted a phantom thumb on a single line. The wrapper owns the
     12.5rem cap (upstream max-h-50); the editor stays unclamped and clips its
-    ink. Scoped to the same >=40rem media as the line box — below that the
-    prompt stays on leading-relaxed and upstream's editor scroll is left alone.
+    ink. The editor's siblings — Lexical's placeholder and spacer — carry
+    overflow clip on both axes (clip, not hidden, so the pure-paint
+    placeholder never becomes a scroll box; both axes, so an unbreakable
+    approval-detail placeholder can neither eat its own 16px line with a
+    horizontal scrollbar nor propagate one onto the wrapper), because the
+    placeholder's overhanging line box otherwise reaches the wrapper's
+    scrollable overflow through its abspos box and paints the thumb on an
+    empty prompt only. Scoped to the same >=40rem media as the
+    line box — below that the prompt stays on leading-relaxed and upstream's
+    editor scroll is left alone.
     The prompt and its placeholder are set by one rule and must stay that way:
     they are two elements stacked exactly on top of each other, and any
     disagreement about size or leading puts the caret beside the text it

apps/web/src/__fork_guards__/forkComposerShell.test.ts

@@ -210,16 +210,44 @@ describe("fork guard: fork-composer-shell", () => {
     expect(scrollport!.body).toMatch(/max-height:\s*12\.5rem/u);
     expect(scrollport!.body).toMatch(/overflow-y:\s*auto/u);
 
+    // The `~ div` exclusion is load-bearing: the sibling clip rule below
+    // satisfies the other three predicates too, so without it this `find`
+    // resolves to the unclamp rule only because that rule happens to sit
+    // earlier in the file. Reorder the two blocks and `max-height: none`
+    // starts interrogating the sibling rule and fails for a reason unrelated
+    // to the invariant — the same class of wrong-thing-satisfied-it bug the
+    // @media prefix match already cost this file.
     const editor = rules.find(
       (rule) =>
         inDesktopLineBoxMedia(rule) &&
         rule.selector.includes("[data-fork-composer-prompt]") &&
-        rule.selector.includes('[data-testid="composer-editor"]'),
+        rule.selector.includes('[data-testid="composer-editor"]') &&
+        !/~\s*div/u.test(rule.selector),
     );
     expect(editor, "desktop editor unclamp rule missing or unscoped").toBeDefined();
     expect(editor!.body).toMatch(/max-height:\s*none/u);
     expect(editor!.body).toMatch(/overflow-y:\s*hidden/u);
 
+    // The editor's clip does not cover its siblings: the placeholder's line
+    // box overhangs its inset-0 box by a pixel at 14/16, and abspos overflow
+    // propagates to the wrapper's scrollable overflow — a thumb that appears
+    // only while the prompt is empty. Clip specifically, not hidden: hidden
+    // computes the other axis's visible to auto, and an unbreakable
+    // approval-detail placeholder would then hand the 16px line to a
+    // horizontal scrollbar. Both axes, because a visible x would propagate
+    // that same text sideways into the wrapper's auto overflow-x instead.
+    // (The editor tolerates hidden only because pre-wrap + wrap-break-word
+    // forecloses horizontal overflow.)
+    const siblings = rules.find(
+      (rule) =>
+        inDesktopLineBoxMedia(rule) &&
+        rule.selector.includes("[data-fork-composer-prompt]") &&
+        /\[data-testid="composer-editor"\]\s*~\s*div/u.test(rule.selector),
+    );
+    expect(siblings, "desktop placeholder clip rule missing or unscoped").toBeDefined();
+    expect(siblings, "the unclamp and clip rules must stay distinct").not.toBe(editor);
+    expect(siblings!.body).toMatch(/overflow:\s*clip/u);
+
     // An imperative attribute toggle is the failure mode this replaces — see
     // .fork/notes/FORK-CUSTOMIZATION-DECISIONS.md#fork-composer-shell.
     expect(theme).not.toContain("data-composer-prompt-scrollable");

apps/web/src/theme.custom.css

@@ -397,6 +397,31 @@
     max-height: none;
     overflow-y: hidden;
   }
+
+  /* The placeholder (and Lexical's zero-height spacer) are the editor's
+     siblings, outside its clip. At 14/16 the placeholder's line box is a pixel
+     taller than its inset-0 box, and that pixel propagates through the
+     absolutely-positioned box into the wrapper's scrollable overflow — a
+     phantom thumb that shows only while the prompt is empty, because typing
+     unmounts the placeholder. Typed text already paints without that pixel.
+
+     `clip`, not `hidden`: setting an axis to hidden computes the other
+     axis's `visible` to `auto`, which turns this pure-paint element into a
+     scroll box — and the approval-state placeholder is server-supplied text
+     (paths, URLs, SHAs) with no break opportunity, so the reserved horizontal
+     scrollbar would eat the entire 16px line. And both axes, not just y: left
+     visible, that same unbreakable text propagates sideways overflow into the
+     wrapper — whose overflow-x computes to auto alongside its overflow-y —
+     and paints a 16px horizontal scrollbar there instead. clip contains the
+     element on both axes without creating a scroll container anywhere. The
+     editor above gets away with hidden only because pre-wrap +
+     wrap-break-word means it can never overflow horizontally. */
+  :root[data-fork="noahhendrickson-t3code"]
+    [data-fork-composer-prompt]
+    [data-testid="composer-editor"]
+    ~ div {
+    overflow: clip;
+  }
 }
 
 /* The in-box pills and the control row below it share one 24px ghost treatment: