scope windows: their own frame pump, and a diagnosis that names the GSK renderer

A second toplevel's frame clock is not the main window's: the tick binds, queues a render and
mirrors the span, but with nothing asking GTK for frames on *that* window the area can sit with
its queue full and present nothing -- a blank plot. Every scope window now carries a tick
callback on its own area (`gtk_gl_area_queue_render` per frame), which is the request that keeps
its frame clock running.

The "cannot paint" diagnosis now names the window's GSK renderer (`[GSK renderer: GskVulkanRenderer]`)
and says to restart with `GSK_RENDERER=gl` when it is the Vulkan one -- the interaction between
the Vulkan GSK renderer and an embedded `GtkGLArea` in a second toplevel is a known trouble spot,
and a tiling WM has no title bar to carry a title, so the sentence goes to stderr and the status
bar instead.

README.md

@@ -149,8 +149,12 @@ the signal page's *Plot on* buttons list **every window**: one group per window
 menu** — y axis (auto or a fixed range), *Fit X axis*, *Fit all*, remove a trace — acting on that
 window's rows (the x fit resets the **shared** time span, so every view follows); a detached
 window's menu carries **Attach back to the main window** beside them. A window that cannot paint
-says so in its own title after two seconds (and on stderr): a blank plot is indistinguishable from
-a context that failed, and the sentence names which.
+says so on stderr and in the status bar after two seconds, naming its **GSK renderer**: a blank
+plot is indistinguishable from a context that failed, and the GTK **Vulkan** renderer is known to
+interact badly with an embedded `GtkGLArea` in a second toplevel — the sentence then says to
+restart with `GSK_RENDERER=gl`. Each scope window also carries a **tick callback of its own**,
+which keeps its toplevel's frame clock advancing: the main window's tick binds and queues the
+render, but a second window whose clock never ticks presents nothing.
 mode or the span.
 
 **The window.** The plot is the window. Under it sits a fixed-height **badge bar** (36 px). Its left