diff --git a/builtins/libAfterImage/draw.c b/builtins/libAfterImage/draw.c
index b1606bd1c3fbe..e4d10bb6fa2b8 100644
--- a/builtins/libAfterImage/draw.c
+++ b/builtins/libAfterImage/draw.c
@@ -340,7 +340,7 @@ apply_tool_point_colored(ASDrawContext *ctx, int curr_x, int curr_y, CARD32 rati
 		dst += curr_y * cw + curr_x;
 		if (get_flags(ctx->flags, ASDrawCTX_UsingScratch))
 		{
-			CARD32 value = (ARGB32_ALPHA8(ctx->tool->matrix[0])*ratio)/255 ;
+			CARD32 value = ratio ;	/* coverage only; alpha applied on merge */
 			if( *dst < value ) 
 				*dst = value ;
 		}		
@@ -391,7 +391,7 @@ fill_hline_notile_colored(ASDrawContext *ctx, int x_from, int y, int x_to, CARD3
 		{
 			while( x1 <= x2 ) 
 			{
-				CARD32 value = (ARGB32_ALPHA8(ctx->tool->matrix[0])*ratio)/255 ;
+				CARD32 value = ratio ;	/* coverage only; alpha applied on merge */
 				if( dst[x1] < value ) 
 					dst[x1] = value ;
 				++x1 ;
diff --git a/graf2d/asimage/CMakeLists.txt b/graf2d/asimage/CMakeLists.txt
index 8f835cc7e16ad..8a0a67e7012ab 100644
--- a/graf2d/asimage/CMakeLists.txt
+++ b/graf2d/asimage/CMakeLists.txt
@@ -53,3 +53,5 @@ ROOT_STANDARD_LIBRARY_PACKAGE(ASImageGui
 )
 
 ROOT_INSTALL_HEADERS()
+
+ROOT_ADD_TEST_SUBDIRECTORY(test)
diff --git a/graf2d/asimage/test/CMakeLists.txt b/graf2d/asimage/test/CMakeLists.txt
new file mode 100644
index 0000000000000..c08327d3e21db
--- /dev/null
+++ b/graf2d/asimage/test/CMakeLists.txt
@@ -0,0 +1,7 @@
+# Copyright (C) 1995-2026, Rene Brun and Fons Rademakers.
+# All rights reserved.
+#
+# For the licensing terms see $ROOTSYS/LICENSE.
+# For the list of contributors see $ROOTSYS/README/CREDITS.
+
+ROOT_ADD_GTEST(TASImageDraw tasimage_draw.cxx LIBRARIES ASImage)
diff --git a/graf2d/asimage/test/tasimage_draw.cxx b/graf2d/asimage/test/tasimage_draw.cxx
new file mode 100644
index 0000000000000..bc8b942e2fb5c
--- /dev/null
+++ b/graf2d/asimage/test/tasimage_draw.cxx
@@ -0,0 +1,68 @@
+#include "gtest/gtest.h"
+
+#include "TASImage.h"
+
+namespace {
+
+constexpr UInt_t kSize = 64;
+
+constexpr UInt_t kPixels = kSize * kSize;
+
+// Index of the four canvas corners.
+constexpr UInt_t kCorners[4] = {0, kSize - 1, (kSize - 1) * kSize, kPixels - 1};
+
+// Draw a filled circle of `colour` well inside a kSize x kSize image and check
+// that the fill stayed inside it. The corner values are compared against what
+// they were before drawing rather than against a constant, so the test does not
+// depend on how a fresh TASImage is initialised.
+void CheckFilledCircleStaysInside(const char *colour)
+{
+   TASImage img(kSize, kSize);
+
+   UInt_t *argb = img.GetArgbArray();
+   ASSERT_NE(argb, nullptr);
+
+   UInt_t before[4];
+   for (int i = 0; i < 4; ++i)
+      before[i] = argb[kCorners[i]];
+   const UInt_t centre = (kSize / 2) * kSize + kSize / 2;
+   const UInt_t centreBefore = argb[centre];
+
+   img.DrawCircle(kSize / 2, kSize / 2, kSize / 4, colour, -1);
+
+   argb = img.GetArgbArray();
+   ASSERT_NE(argb, nullptr);
+
+   for (int i = 0; i < 4; ++i)
+      EXPECT_EQ(argb[kCorners[i]], before[i]) << "the fill escaped the circle and reached corner " << i;
+
+   EXPECT_NE(argb[centre], centreBefore) << "the circle was not filled at all";
+}
+
+} // namespace
+
+// https://github.com/root-project/root/issues/23014
+//
+// libAfterImage scaled the coverage it wrote into the scratch canvas by the
+// brush alpha, which made the flood fill that closes a filled shape depend on
+// that alpha. Two symptoms followed, and this geometry shows both: at an alpha
+// of 0x8C or below the fill never terminated, and between 0x8D and 0xFE it
+// returned but leaked through the anti-aliased outline and covered the whole
+// image. Only a fully opaque brush behaved correctly.
+
+TEST(TASImage, FilledCircleOpaque)
+{
+   CheckFilledCircleStaysInside("#FFFF0000");
+}
+
+// Used to leak out of the circle and fill the whole image.
+TEST(TASImage, FilledCircleHighAlpha)
+{
+   CheckFilledCircleStaysInside("#C0FF0000");
+}
+
+// Used to hang: the colour from the issue report.
+TEST(TASImage, FilledCircleSemiTransparent)
+{
+   CheckFilledCircleStaysInside("#7FFF0000");
+}