What happened?

Dragging the waterfall to pan (horizontal frequency scroll) is choppy again on high-resolution / wide displays, with the UI thread stalling ~60–80 ms per pan step. By inspection this traces to #3668 (clean-room KiwiSDR integration), merged 2026-06-20 — the same day. It re-introduces, by a different mechanism, the per-pan-step waterfall cost that #3578 had removed (which itself fixed the same class of choppiness, #3575, on wide displays).

Root cause

#3578 made per-pan reproject cheap: the waterfall history image is no longer globally reprojected — each history row records its own capture frame and only the visible viewport is remapped.

#3668 changed SpectrumWidget::handleWaterfallFrequencyFrameChange() from a single reprojectWaterfall() into an unconditional native + kiwi double reproject, each wrapped in a waterfall-stream-state save/restore:

reprojectStream(false);   // native
reprojectStream(true);    // kiwi — runs every pan step even with no KiwiSDR receiver

On every pan mouse-move, even when KiwiSDR is inactive, this costs two things:

1. A second, wasted reproject pass for a stream with no on-screen data.
2. saveCurrentWaterfallStreamState() does state.waterfall = m_waterfall — a QImage assignment, so the buffer is now shared (copy-on-write). The next m_waterfall.fill(Qt::black) inside rebuildWaterfallViewportForFrame() therefore detaches = deep-copies the entire waterfall image (large on wide/high-DPI displays) on every pan step. That deep-copy is exactly the per-step cost perf(spectrum): per-row waterfall-history frequency frames (no per-pan reproject) #3578 had eliminated.

Measured (aether.perf, same telemetry as #3578)

i9-13900K / RTX 4090, FLEX-6600, wide high-resolution panadapter, D3D11/GPU path. Same binary; one env var (AETHER_WF_KIWI_ALWAYS) toggles only the second pass. During a continuous hand pan:

fftRestarts/wfRestarts = 0 and packet loss = 0 in both runs — the radio/stream is not involved. renderP95 ~7 ms in both — the GPU render is not the bottleneck. The cost is purely the second pass + the deep-copy.

What did you expect?

Pan to stay smooth (as after #3578), regardless of whether a KiwiSDR receiver is connected.

Steps to reproduce

1. Connect to a FLEX radio; open a panadapter with a waterfall on a wide / high-resolution display.
2. Grab the waterfall and pan (drag horizontally) continuously for a few seconds.
3. Panning is choppy/laggy. With aether.perf enabled (Help → Support → "Performance"), wfUpdateP95Ms spikes to ~60–80 ms and panAgeP95Ms to ~150–170 ms during the drag.

A/B from one binary: launch with AETHER_WF_KIWI_ALWAYS=1 → stall reproduces; without it (active-stream-only) → stall gone.

AetherSDR version

26.6.3

Radio model & firmware

FLEX-6600

Operating system

Windows

OS version and hardware

Windows 11 (build 26200), Qt 6.8.3 msvc2022_64; i9-13900K / RTX 4090, wide high-resolution panadapter.

Fix PR incoming (reproject only the active stream; remap the other on stream-switch). Couldn't self-assign — this account has no triage permission here.