feat(tabs): polish new-chat button — stuck-only square edge, New Chat labelHEADmain

- isStuckToEnd (pure): square the sticky '+' right edge only while it floats
  over scrolled tabs; rounded at rest. Edge-measured in TabBar via a disposed
  scroll + ResizeObserver effect (RO guarded for non-browser envs).
- Show a temporary 'New Chat' title when the draft is selected, with the '+'
  moved to the trailing close-button slot for consistency with real tabs.

1 files changed, 24 insertions, 0 deletions

diff --git a/src/features/tabs/tabs.test.ts b/src/features/tabs/tabs.test.ts
index 3034e76..5effa9b 100644
--- a/src/features/tabs/tabs.test.ts
+++ b/src/features/tabs/tabs.test.ts
@@ -6,6 +6,7 @@ import {
 	createTab,
 	deriveTitle,
 	initialState,
+	isStuckToEnd,
 	newDraft,
 	selectTab,
 	setModel,
@@ -189,3 +190,26 @@ describe("deriveTitle", () => {
 		expect(result).toBe(`${"a".repeat(40)}\u2026`);
 	});
 });
+
+describe("isStuckToEnd", () => {
+	it("is false when the strip does not overflow", () => {
+		expect(isStuckToEnd({ scrollLeft: 0, clientWidth: 500, scrollWidth: 500 })).toBe(false);
+		expect(isStuckToEnd({ scrollLeft: 0, clientWidth: 500, scrollWidth: 400 })).toBe(false);
+	});
+
+	it("is true when overflowing and scrolled to the left", () => {
+		expect(isStuckToEnd({ scrollLeft: 0, clientWidth: 500, scrollWidth: 1000 })).toBe(true);
+	});
+
+	it("is true when overflowing and scrolled to the middle", () => {
+		expect(isStuckToEnd({ scrollLeft: 250, clientWidth: 500, scrollWidth: 1000 })).toBe(true);
+	});
+
+	it("is false when overflowing but scrolled fully to the right", () => {
+		expect(isStuckToEnd({ scrollLeft: 500, clientWidth: 500, scrollWidth: 1000 })).toBe(false);
+	});
+
+	it("treats a 1px subpixel gap at the end as at-rest (epsilon)", () => {
+		expect(isStuckToEnd({ scrollLeft: 499, clientWidth: 500, scrollWidth: 1000 })).toBe(false);
+	});
+});