perf(charts): fill chart areas in one linear pass

The filled sparklines and the market price chart built their area fill
with PathFillConcave, which runs a per-scanline pass over the whole
filled region. For a simple area under an x-monotone curve that work is
unnecessary.

- A new helper fills the area between an x-monotone polyline and a
  horizontal baseline directly as a triangle strip, in one pass over the
  points. The market sparklines, the market price chart and the mining
  hashrate sparkline all use it. The fill is flat and non-anti-aliased;
  the anti-aliased curve drawn on top hides the stepped top edge, and
  the faint side and bottom edges are imperceptible at these alphas.

Tests: the strip's vertex and index counts, index bounds, and that its
triangle area matches the trapezoid area under the polyline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/material/draw_helpers.h

@@ -9,6 +9,7 @@
 #include "tooltip_style.h"
 #include "scroll_edge_fade.h"
 #include "rect_cull.h"
+#include "fill_strip.h"
 #include "../layout.h"
 #include "../schema/element_styles.h"
 #include "../schema/color_var_resolver.h"

src/ui/material/fill_strip.h

@@ -0,0 +1,38 @@
+#pragma once
+#include "imgui.h"
+
+namespace dragonx { namespace ui { namespace material {
+
+inline int FillStripVertexCount(int n) { return n < 2 ? 0 : 2 * n; }
+inline int FillStripIndexCount(int n)  { return n < 2 ? 0 : 6 * (n - 1); }
+
+/// Index list for the strip: vertices come in (top_i, bottom_i) pairs, 2 triangles per column.
+/// `base` is the first vertex index; `out` must hold FillStripIndexCount(n) entries.
+inline void BuildFillStripIndices(int n, unsigned base, unsigned* out) {
+    for (int i = 0; i + 1 < n; ++i) {
+        const unsigned a = base + 2u * (unsigned)i, b = a + 1, c = a + 2, d = a + 3;
+        *out++ = a; *out++ = b; *out++ = d;
+        *out++ = a; *out++ = d; *out++ = c;
+    }
+}
+
+/// Fills the area between an x-monotone polyline and the horizontal line y = baseY with a flat colour,
+/// in O(n). Non-anti-aliased: draw the AA polyline on top (it hides the stepped top edge; the side/bottom
+/// edges are imperceptible at the ~10% alpha these fills use). Not for concave/self-overlapping shapes.
+inline void FillUnderPolyline(ImDrawList* dl, const ImVec2* pts, int n, float baseY, ImU32 col) {
+    if (n < 2 || (col & IM_COL32_A_MASK) == 0) return;
+    const ImVec2 uv = ImGui::GetFontTexUvWhitePixel();
+    dl->PrimReserve(FillStripIndexCount(n), FillStripVertexCount(n));
+    const unsigned base = dl->_VtxCurrentIdx;          // read AFTER PrimReserve (it may roll the vertex offset)
+    for (int i = 0; i < n; ++i) {
+        dl->PrimWriteVtx(pts[i], uv, col);
+        dl->PrimWriteVtx(ImVec2(pts[i].x, baseY), uv, col);
+    }
+    for (int i = 0; i + 1 < n; ++i) {
+        const unsigned a = base + 2u * (unsigned)i;
+        dl->PrimWriteIdx((ImDrawIdx)a);       dl->PrimWriteIdx((ImDrawIdx)(a + 1)); dl->PrimWriteIdx((ImDrawIdx)(a + 3));
+        dl->PrimWriteIdx((ImDrawIdx)a);       dl->PrimWriteIdx((ImDrawIdx)(a + 3)); dl->PrimWriteIdx((ImDrawIdx)(a + 2));
+    }
+}
+
+}}} // namespace

src/ui/windows/market_tab.cpp

@@ -183,10 +183,7 @@ static void pfDrawSparklineFilled(ImDrawList* dl, ImVec2 mn, ImVec2 mx,
     std::vector<ImVec2> pts;
     if (!pfProjectSeries(hist, mn, mx, pts)) return;
     int n = (int)pts.size();
-    for (int i = 0; i < n; i++) dl->PathLineTo(pts[i]);
-    dl->PathLineTo(ImVec2(pts[n - 1].x, mx.y));
-    dl->PathLineTo(ImVec2(pts[0].x, mx.y));
-    dl->PathFillConcave(WithAlpha(lineCol, 28));
+    FillUnderPolyline(dl, pts.data(), n, mx.y, WithAlpha(lineCol, 28));
     dl->AddPolyline(pts.data(), n, WithAlpha(lineCol, 220), ImDrawFlags_None, 1.4f * dp);
 }
 
@@ -1826,10 +1823,7 @@ static void mktDrawPriceChart(const MktCtx& cx)
                 points[i] = ImVec2(plotLeft + t * plotW, yOf(s_mkt.history[i]));
             }
             // Flat translucent area fill under the curve (matches the portfolio group sparklines).
-            for (size_t i = 0; i < n; i++) dl->PathLineTo(points[i]);
-            dl->PathLineTo(ImVec2(points[n - 1].x, plotBottom));
-            dl->PathLineTo(ImVec2(points[0].x, plotBottom));
-            dl->PathFillConcave(WithAlpha(dirCol, 28));
+            FillUnderPolyline(dl, points.data(), (int)n, plotBottom, WithAlpha(dirCol, 28));
             // Line (no per-point dots — a clean curve).
             dl->AddPolyline(points.data(), (int)points.size(), lineCol, ImDrawFlags_None, S.drawElement("tabs.market", "chart-line-thickness").size);
         }

src/ui/windows/mining_stats.cpp

@@ -102,11 +102,7 @@ static void DrawHashrateSparkline(ImDrawList* dl, ImVec2 regionMin, ImVec2 regio
     }
 
     if (points.size() >= 2) {
-        for (size_t i = 0; i < points.size(); i++)
-            dl->PathLineTo(points[i]);
-        dl->PathLineTo(ImVec2(points.back().x, plotBottom));
-        dl->PathLineTo(ImVec2(points.front().x, plotBottom));
-        dl->PathFillConcave(WithAlpha(Success(), 25));
+        FillUnderPolyline(dl, points.data(), (int)points.size(), plotBottom, WithAlpha(Success(), 25));
     }
 
     dl->AddPolyline(points.data(), (int)points.size(), WithAlpha(Success(), 200),