fix

rfg_adc_plotter/gui/pyqtgraph_backend.py

@@ -1640,6 +1640,7 @@ def run_pyqtgraph(args) -> None:
         changed = runtime.ring.ensure_init(sweep_width)
         if not changed:
             return
+        log_debug_event("ring_resize", f"ring resized to width {int(sweep_width)}", every=1)
         f_min = float(runtime.range_min_ghz)
         f_max = float(runtime.range_max_ghz)
         freq_bounds = resolve_axis_bounds(runtime.current_freqs)
@@ -1657,18 +1658,11 @@ def run_pyqtgraph(args) -> None:
             padding=0,
         )
         set_x_range_if_changed("line_x", p_line, f_min, f_max, padding=0)
-        disp_fft = fft_bscan_image_to_db(runtime.ring.get_display_fft_linear())
-        if disp_fft is not None:
-            img_fft.setImage(disp_fft, autoLevels=False)
-        set_image_rect_if_changed("fft_waterfall_rect", img_fft, 0.0, 0.0, float(max_sweeps), 1.0)
-        set_xy_range_if_changed(
-            "fft_waterfall_range",
-            p_spec,
-            x_bounds=(0, max_sweeps - 1),
-            y_bounds=(0.0, 1.0),
-            padding=0,
-        )
-        set_x_range_if_changed("fft_x", p_fft, 0.0, 1.0, padding=0)
+        # NOTE: do NOT reset the B-scan (fft_waterfall) rect/range to a (0,1)
+        # placeholder here. The real distance geometry (0..~12 m) is applied every
+        # frame by update_physical_axes() and the B-scan draw block using the stable
+        # axis. Writing the placeholder on every ring resize made the B-scan y-axis
+        # thrash between height 1 and 12 → the flicker.
 
     def _active_distance_axis() -> Optional[np.ndarray]:
         if runtime.current_distances is not None and runtime.current_distances.size > 0:

rfg_adc_plotter/state/ring_buffer.py

@@ -10,6 +10,13 @@ import numpy as np
 from rfg_adc_plotter.constants import FFT_LEN, SWEEP_FREQ_MAX_GHZ, SWEEP_FREQ_MIN_GHZ
 from rfg_adc_plotter.processing.fft import compute_distance_axis, compute_fft_mag_row, fft_mag_to_db
 
+# The device sweep length jitters by a few percent (e.g. 896..920). Only treat a
+# shrink as real when the new width drops below this fraction of the current width;
+# genuine format changes (e.g. 2048 -> 256, or halving) fall well below it. This
+# stops the ring from reallocating every frame — which churned the buffer and made
+# the B-scan flicker.
+RING_WIDTH_SHRINK_FRACTION = 0.8
+
 
 class RingBuffer:
     """Store raw sweeps, FFT rows, and matching time markers."""
@@ -103,7 +110,12 @@ class RingBuffer:
             self.ring_fft_input = np.full((self.max_sweeps, self.width), np.nan + 0j, dtype=np.complex64)
             self.head = 0
             changed = True
-        elif target_width != self.width:
+        elif target_width > self.width or target_width < int(self.width * RING_WIDTH_SHRINK_FRACTION):
+            # Resize when the sweep grows, or shrinks by a meaningful fraction (a real
+            # format change). Ignore small shrinks: the device sweep length jitters by
+            # a few percent (e.g. 896 vs 920). Resizing on every such shrink churned
+            # the ring every frame and made the B-scan flicker. A shorter sweep just
+            # fills fewer columns (the rest stays NaN); the width converges to the max.
             new_ring = np.full((self.max_sweeps, target_width), np.nan, dtype=np.float32)
             new_fft_input = np.full((self.max_sweeps, target_width), np.nan + 0j, dtype=np.complex64)
             take = min(self.width, target_width)