fix(chat): stabilize Facebook keyboard fallback

src/app/chat/hooks/__tests__/chat-keyboard-geometry.test.ts

@@ -9,6 +9,7 @@ import {
 
 const baseline: ChatKeyboardViewportBaseline = {
   layoutHeight: 800,
+  layoutWidth: 390,
   visualHeight: 760,
 };
 
@@ -82,9 +83,42 @@ describe("resolveChatKeyboardInset", () => {
 });
 
 describe("resolveChatKeyboardFallbackInset", () => {
-  it("scales with viewport height and remains bounded", () => {
-    expect(resolveChatKeyboardFallbackInset(500)).toBe(276);
-    expect(resolveChatKeyboardFallbackInset(800)).toBe(352);
-    expect(resolveChatKeyboardFallbackInset(1_200)).toBe(396);
+  it.each([
+    [{ layoutWidth: 320, layoutHeight: 568 }, 256],
+    [{ layoutWidth: 360, layoutHeight: 640 }, 285],
+    [{ layoutWidth: 390, layoutHeight: 844 }, 316],
+    [{ layoutWidth: 430, layoutHeight: 932 }, 347],
+  ])("fits common viewport %o", (viewport, expectedInset) => {
+    expect(resolveChatKeyboardFallbackInset(viewport)).toBe(expectedInset);
+  });
+
+  it("stays stable across tall viewports with the same width", () => {
+    expect(
+      resolveChatKeyboardFallbackInset({
+        layoutWidth: 390,
+        layoutHeight: 760,
+      }),
+    ).toBe(316);
+    expect(
+      resolveChatKeyboardFallbackInset({
+        layoutWidth: 390,
+        layoutHeight: 900,
+      }),
+    ).toBe(316);
+  });
+
+  it("keeps the keyboard estimate within its bounds", () => {
+    expect(
+      resolveChatKeyboardFallbackInset({
+        layoutWidth: 300,
+        layoutHeight: 400,
+      }),
+    ).toBe(256);
+    expect(
+      resolveChatKeyboardFallbackInset({
+        layoutWidth: 600,
+        layoutHeight: 1_000,
+      }),
+    ).toBe(356);
   });
 });