fix(plot,glfw): layer the crosshair under the handles, sync it across a figure, and harden the lookup

src/plot/plot.cpp

@@ -1162,23 +1162,33 @@ CrosshairSpan crosshair_span(const data::Series& series, f64 x) noexcept {
   if (x < x0 || x > newest) return span;
 
   // Index arithmetic, not a search: the grid is uniform, so the sample at or before `x` is
-  // `floor((x - x0) / dx)` and there is nothing to look up. Two special cases keep the span
-  // honest at the ends and under rounding -- an index past the last sample collapses onto
-  // it, and a `t` that rounds up to 1.0 (a query one ulp above a sample) collapses onto the
-  // sample it has reached rather than reporting an interpolation of width zero.
+  // `floor((x - x0) / dx)` and there is nothing to look up. The two bounds are checked
+  // *before* the cast, which is also what keeps it defined: `index` is a quotient of two
+  // caller-supplied doubles and can overflow to infinity for an extreme origin and spacing,
+  // and casting that to `u64` is undefined. `!(index > 0.0)` is the NaN test as well.
   const f64 index = (x - x0) / dx;
-  const u64 i = index <= 0.0 ? 0u : static_cast<u64>(std::floor(index));
-  if (i >= n - 1u) {
-    span.begin = span.end = n - 1u;
+  if (!(index > 0.0)) {
+    span.begin = span.end = 0u;  // the oldest sample, or a rounding step below it
     span.t = 0.0;
     span.valid = true;
     return span;
   }
+  if (index >= static_cast<f64>(n - 1u)) {
+    span.begin = span.end = n - 1u;  // the newest sample, or a step past it
+    span.t = 0.0;
+    span.valid = true;
+    return span;
+  }
+  // `index` is finite and inside `(0, n-1)`, so truncating it is a floor on a value a `u64`
+  // holds. Two rounding cases keep the span honest at a sample: a `t` that lands on 0 is the
+  // query sitting *on* sample `i`, and one that rounds up to 1.0 is it sitting on the sample
+  // after it -- neither is an interpolation with no width.
+  const u64 i = static_cast<u64>(index);
   const f64 t = index - static_cast<f64>(i);
   if (!(t > 0.0)) {
-    span.begin = span.end = i;  // exactly on sample `i`
+    span.begin = span.end = i;
   } else if (t >= 1.0) {
-    span.begin = span.end = i + 1u;  // exactly on the sample after it
+    span.begin = span.end = i + 1u;
   } else {
     span.begin = i;
     span.end = i + 1u;