D889 — The graph draws on demand, and a network too costly to tick rests its drift

Decision. The 3D Atlas draws a frame only when something moved or changed: a simulation tick, a camera move (the reader's, its damping, the panorama), an animation (bloom, morph, fades, signals) or a change the frame loop cannot see, which the scene marks with invalidate() (new data or colours, a camera goal, a frame listener, a tab coming back). Draws are at most about 60 a second whatever the display's refresh. The live simulation's drift continues only while one tick costs at most a quarter of a 60 Hz frame (DRIFT_TICK_BUDGET_MS, 4 ms, resumed below 3 ms); past it the network settles and stops ticking as with motion off, while the panorama and the signals go on. The cost is measured as the scene ticks, warm-up included, so the decision follows the graph on screen and the machine it runs on.

Why. At 5,000 concepts one tick costs ~23 ms on an Apple M5: the always-on drift of D234 took a full core and still could not hold 60 fps, and a static graph with motion off was redrawn at the display's 144 Hz for an identical picture (#861's idle measurement). The breathing network stays what D234 made it on the graphs where it is affordable; on the large ones the reader keeps a turning, responsive picture instead of a hot machine.

Alternatives rejected. - Lower the tick rate with the graph's size: a 23 ms tick a few times a second is a visible hitch in every rotating frame, and the drift slows to nothing. - A fixed node-count threshold: the cost depends on the machine and on the links as much as on the count; measuring it answers both. - Breathing as a draw-time offset with the simulation cooled: cheaper, but it is a second motion model to keep in step with the physics for one effect.

Consequences. A new way to change the picture from outside the frame loop must call invalidate(), or it shows only with the next movement. The video export (renderFrame) always draws and never adapts its drift, so its frames do not depend on the exporting machine. sustainsDrift is the one place the budget lives.