refactor(overlay): derive histogram quad slots, fix stale geometry comments

openxr-api-layer/shaders/overlay_bars_ps.hlsl

@@ -64,7 +64,7 @@ float4 PSMain(VSOutput i) : SV_TARGET
     // a bar chart and removes that artifact; the shared baseline at the
     // strip floor stays crisp for the same reason.
     //
-    // CONTRACT: bar geometry stays integer-aligned (4-px width, integer
+    // CONTRACT: bar geometry stays integer-aligned (5-px width, integer
     // xLeft — see HistogramBarRenderer::drawPanel). With that, covX
     // collapses to 1 everywhere and the sides ALSO render crisp — the
     // bars look pixel-perfect uniform. If a future layout reintroduces

openxr-api-layer/utils/overlay_renderer.cpp

@@ -2464,12 +2464,17 @@ namespace openxr_api_layer::detail {
             // bars; the count derives from kQuadSlots so adding a slot can't
             // silently desync it. kSlotGridEnd (one past the grid run) stays
             // separate from the budget slot that immediately follows it.
-            static constexpr UINT kSlotGridFirst = 1;  // interior gridlines [1..4]
+            static constexpr UINT kSlotGridFirst = 1;  // interior gridlines [kSlotGridFirst, kSlotGridEnd)
             static constexpr UINT kSlotGridEnd   = 5;  // one past the last grid slot
-            static constexpr UINT kSlotBudget    = 5;  // budget line
-            static constexpr UINT kSlotBaseline  = 6;  // 0-ms baseline
-            static constexpr UINT kSlotAxis      = 7;  // left vertical ms-axis
-            static constexpr UINT kQuadSlots     = 8;  // total
+            // Trailing single-quad slots derive sequentially from kSlotGridEnd
+            // so they can never silently collide with the grid run: bump
+            // kSlotGridEnd (e.g. to add an interior gridline) and budget /
+            // baseline / axis / total all shift up in lockstep instead of one
+            // quietly overwriting a grid slot. Values are unchanged (5/6/7/8).
+            static constexpr UINT kSlotBudget    = kSlotGridEnd;       // budget line
+            static constexpr UINT kSlotBaseline  = kSlotBudget + 1;    // 0-ms baseline
+            static constexpr UINT kSlotAxis      = kSlotBaseline + 1;  // left vertical ms-axis
+            static constexpr UINT kQuadSlots     = kSlotAxis + 1;      // total
             // The [kSlotGridFirst, kSlotGridEnd) gridline slots must cover every
             // non-zero tick (kMaxMsAxisTicks counts the 0 tick, drawn as the
             // baseline rather than an interior line). Catches a kMaxMsAxisTicks

openxr-api-layer/utils/overlay_renderer.h

@@ -171,10 +171,13 @@ namespace openxr_api_layer::detail {
     // code can declare matching rings without hard-coding the value
     // (and the cpp's static_assert below guards against drift).
     //
-    // 133 = the bar count that fills the GPU histogram strip exactly
-    // with the fixed 4-px-bar / 1-px-gap layout: 133×4 + 132×1 = 664 px
-    // (the strip's inner width). Picking it leaves zero margin and keeps
-    // every bar pixel-aligned. ~133 samples ≈ 1.5 s @ 90 Hz.
+    // 133 = the histogram ring size. Originally chosen so the strip filled
+    // exactly with 4-px bars + 1-px gaps (133×4 + 132×1 = 664 px). The bars
+    // are now 5-px wide at a 5-px step with no gap (see overlay_renderer.cpp),
+    // so 133 bars span 133×5 = 665 px and the .cpp centres or left-clips the
+    // run within the strip's actual width rather than filling it exactly. The
+    // count is retained for the time window it captures: ~133 samples ≈
+    // 1.5 s @ 90 Hz.
     constexpr std::size_t kOverlayHistoRingSize = 133;
 
     // -------- GPU-path snapshot entry point (Task 18) ---------------------