tooltip: clamp box width to client on narrow panels; add narrow-client test

tests/test_tooltip.cpp

@@ -93,6 +93,20 @@ static void testBothClipClampsBottom() {
     CHECK(tb.y == 2);
 }
 
+// --- Narrow client: box width clamped ----------------------------------------
+
+// When the client is narrower than the box's natural width, the box is shrunk to fit and
+// the result satisfies x >= margin AND x + width <= clientW - margin.
+// Natural boxW = 80+12 = 92. Narrow client = 60. maxBoxW = 60-4 = 56.
+// Clamped boxW = 56. x = 2 + (108-56)/2 ... centred but clamped to [2, 60-2-56] = [2, 2].
+static void testNarrowClientBoxClamped() {
+    const TooltipSpec spec{};  // margin = 2
+    const TooltipBox tb = computeTooltip(2, 10, 108, 34, 80, 14, 60, 300, spec);
+    CHECK(!tb.empty());
+    CHECK(tb.x >= spec.margin);
+    CHECK(tb.x + tb.width <= 60 - spec.margin);
+}
+
 // --- Degenerate --------------------------------------------------------------
 
 static void testDegenerateEmpty() {
@@ -113,6 +127,7 @@ int main() {
     testClampLeft();
     testFlipAbove();
     testBothClipClampsBottom();
+    testNarrowClientBoxClamped();
     testDegenerateEmpty();
 
     if (g_fail == 0) std::printf("tooltip: all tests passed\n");