diff --git a/common_libs/tests/measure_strafe_heat_display.nim b/common_libs/tests/measure_strafe_heat_display.nim
index be43025..81fb4b6 100644
--- a/common_libs/tests/measure_strafe_heat_display.nim
+++ b/common_libs/tests/measure_strafe_heat_display.nim
@@ -68,6 +68,8 @@ proc main() =
   var m = initStrafe()
   m.debugGraphics = false
 
+  const DrawWindow = 400   ## ticks drawn: enough for ~30 picks, cheap to render
+
   var
     ticks        = 0
     targetTicks  = 0
@@ -86,10 +88,33 @@ proc main() =
     bandComfy    = 0    ## <= 2.5 (half of half)
     bandLow      = 0    ## 2.5 .. 5.0
     bandMarginal = 0    ## 5.0 .. 10.0
+    svgTicks     = 0    ## ticks whose overlay was rendered and inspected
+    svgBad       = 0    ## the chosen tile's heat NUMBER is not in the SVG
+    svgNoPlate   = 0    ## the chosen tile's dark value plate is not in the SVG
+
+  for tick, ws in states:
+    # The GUI half: render the overlay for the first `DrawWindow` ticks, with
+    # the heat grid ALTERNATING on/off so BOTH requirement-D arms are exercised
+    # (with the grid hidden the chosen tile's number must still be there).
+    # Drawing is read-only, so the pick sequence stays byte-identical; the SVG
+    # buffer is cleared every tick exactly as the framework does.
+    m.debugGraphics = tick < DrawWindow
+    if m.debugGraphics:
+      StrafeHeatGrid = (tick mod 2 == 0)
+      clearGraphics()
 
-  for ws in states:
     discard m.computeMove(ws)
     inc ticks
+
+    if m.debugGraphics:
+      let svg = svgOutput()
+      clearGraphics()
+      if m.targetValid and svg.len > 0:
+        inc svgTicks
+        # The number printed ON the chosen tile, as the SVG sees it.
+        if not svg.contains(fmt">{m.targetPathHeat:.1f}</text>"): inc svgBad
+        if not svg.contains("fill=\"#000000\""): inc svgNoPlate
+
     if not m.targetValid: continue
     inc targetTicks
 
@@ -136,6 +161,10 @@ proc main() =
         fmt"({under} under / {over} over, {labelBad} bad)", labelBad == 0
   check fmt"`strafeHeatText` carries the displayed number ({textBad} bad)",
         textBad == 0
+  check fmt"the chosen tile's heat NUMBER is rendered on the tile, grid ON " &
+        fmt"and OFF ({svgTicks} rendered ticks, {svgBad} missing)", svgBad == 0
+  check fmt"the chosen tile's value plate is rendered ({svgNoPlate} missing)",
+        svgNoPlate == 0
   check "the stream contains picks to check", picks > 0
 
   echo ""
@@ -146,6 +175,7 @@ proc main() =
        fmt"max={maxH:.1f}  best-alt known={altKnown}/{picks}"
   echo fmt"buckets: <=2.5 {bandComfy} | 2.5-5 {bandLow} | 5-10 {bandMarginal} | " &
        fmt">10 (forced) {over}"
+  echo fmt"overlay rendered on {svgTicks} ticks (heat grid alternating ON/OFF)"
   echo fmt"sample log fields: [{strafeHeatText(7.3, 9.8)}]  " &
        fmt"[{strafeHeatText(11.4, -1.0)}]"
   echo ""