Skip unchanged realtime FSP redraws

Cache destination bounds and timestamp/value bytes on `Viz`.
Skip identical quote-driven refreshes, including stable NaNs,
while changed samples and forced history repairs still redraw.
Add a regression covering each invalidation path.

(this commit msg was generated in some part by `codex` using
`gpt-6` (`openai`))

piker/ui/_dataviz.py

@@ -319,6 +319,7 @@ class Viz(Struct):
     _in_ds: bool = False
     _index_step: float | None = None
     _time_step: float | None = None
+    _last_fsp_update_sig: tuple[int, int, bytes]|None = None
 
     # map from uppx -> (downsampled data, incremental graphics)
     _src_r: Renderer | None = None

piker/ui/_fsp.py

@@ -101,10 +101,36 @@ def update_fsp_chart(
         )
         return
 
+    # `update_fsp_chart()` runs for every source quote, while an FSP can
+    # publish more slowly. `Viz._last_fsp_update_sig` combines the
+    # destination's absolute half-open `ShmArray` bounds with bitwise
+    # `last_row['time']` and `last_row[array_key]` scalars. Comparing
+    # bytes keeps repeated NaNs stable; scalar equality treats every NaN
+    # as changed. A changed `ShmArray._first.value` catches history
+    # prepends, a changed `ShmArray._last.value` catches sample appends,
+    # and changed row bytes catch mutation of the current sample.
+    # `force_redraw` bypasses this gate because an equal-bound gap repair
+    # can rewrite earlier rows without changing the current row. This is
+    # only a repaint key, not synchronization proof; a mixed SHM read
+    # merely causes one extra later redraw.
+    update_sig: tuple[int, int, bytes] = (
+        shm._first.value,
+        shm._last.value,
+        np.asarray(last_row['time']).tobytes()
+        + np.asarray(last_row[array_key]).tobytes(),
+    )
+    if (
+        not force_redraw
+        and
+        viz._last_fsp_update_sig == update_sig
+    ):
+        return
+
     # update graphics
     # NOTE: this does a length check internally which allows it
     # staying above the last row check below..
     viz.update_graphics(force_redraw=force_redraw)
+    viz._last_fsp_update_sig = update_sig
 
     # XXX: re: ``array_key``: fsp func names must be unique meaning we
     # can't have duplicates of the underlying data even if multiple

tests/test_fsp_sync.py

@@ -33,7 +33,10 @@ from piker.data._sharedmem import NDTokenMsg
 from piker.data.ticktools import FeedQuote
 from piker.fsp._api import Fsp
 from piker.ui._chart import LinkedSplits
-from piker.ui._fsp import update_fsp_vizs
+from piker.ui._fsp import (
+    update_fsp_chart,
+    update_fsp_vizs,
+)
 from tractor.ipc._shm import (
     NDToken,
     ShmArray,
@@ -502,6 +505,58 @@ class Viz:
         self.force_redraws.append(force_redraw)
 
 
+def test_fsp_chart_skips_unchanged_realtime_state() -> None:
+    '''
+    Avoid repainting an unchanged current FSP datum on every quote.
+
+    The display loop calls `update_fsp_chart()` for every realtime
+    quote, even when a given FSP destination has not advanced or
+    changed its current value. That rebuilt and painted the same last
+    step rectangle repeatedly. Exercise identical state, an in-place
+    value update, a new sample with the same value, and a forced
+    history repair. The update counts prove unchanged requests are
+    skipped while each legitimate invalidation still redraws. A final
+    stable-NaN pair proves scalar NaN inequality can not defeat the
+    guard.
+
+    '''
+    shm = FspShm()
+    viz = Viz('flow', shm)
+
+    update_fsp_chart(viz, 'flow', 'flow')
+    update_fsp_chart(viz, 'flow', 'flow')
+    assert viz.updates == 1
+
+    shm._array['flow'][shm._last.value - 1] = 42
+    update_fsp_chart(viz, 'flow', 'flow')
+    assert viz.updates == 2
+
+    shm._array['flow'][shm._last.value] = 42
+    shm._last.value += 1
+    update_fsp_chart(viz, 'flow', 'flow')
+    assert viz.updates == 3
+
+    update_fsp_chart(
+        viz,
+        'flow',
+        'flow',
+        force_redraw=True,
+    )
+    assert viz.updates == 4
+
+    shm._array['flow'][shm._last.value - 1] = np.nan
+    update_fsp_chart(viz, 'flow', 'flow')
+    update_fsp_chart(viz, 'flow', 'flow')
+    assert viz.updates == 5
+    assert viz.force_redraws == [
+        False,
+        False,
+        False,
+        True,
+        False,
+    ]
+
+
 def test_fsp_history_update_redraws_only_derived_vizs() -> None:
     '''
     Keep a source-history prepend from refreshing the whole chart.