improved measuraments ,cursors, zooming and user interactions

src/channel.cpp

@@ -80,14 +80,14 @@ f32 ChannelBuffer::time_of(u64 absolute_index) const noexcept {
   // capacities are equal, so there is no slack at all): once the ring saturates, the
   // few oldest samples of the window routinely point at slots the producer has
   // overwritten since. Returning 0 for those put a stamp of 0 into `x_bounds()`, and
-  // from there into `dx()` and `x0()` -- which is the renderer's entire index
-  // mapping, `(x - x0()) / dx()`. So *one* unreadable slot at the front inflated
-  // `dx()` by the ratio of the whole elapsed stream time to the retained window and
-  // pushed every index up with it: `first` landed well past the first visible
-  // sample, and the samples in between were in the buffer, readable by the crosshair
-  // (`x_at()`/`neighbours()` go to the ring per sample and never saw the sentinel),
-  // and never drawn. That is the "live trace is clipped, only the most recent part
-  // is visible, but the crosshair still reads values there" defect.
+  // from there into `dx()`/`x0()` and every consumer of the window's extent. So
+  // *one* unreadable slot at the front inflated `dx()` by the ratio of the whole
+  // elapsed stream time to the retained window: the axis in `x_bounds()` read the
+  // stream's start rather than the window's, every measurement cut computed from it
+  // came out too wide, and the readout disagreed with what the crosshair said
+  // (`x_at()`/`neighbours()` go to the ring per sample and never saw the sentinel).
+  // That is the "live trace is clipped, only the most recent part is visible, but
+  // the crosshair still reads values there" defect.
   //
   // Clamping degrades the way the library's own `StreamSeries` documents for this
   // contract: a snapshot bounds the sample *count*, not the slots, so the oldest few