[LyX/master] Fix the model of mixing two alpha values

[Koji Yokota]

commit f3500e03ae6410248524975336749eacc6fc9cc0
Author: Koji Yokota <yokota at lyx.org>
Date:   Thu Jun 11 19:16:25 2026 +0900

    Fix the model of mixing two alpha values
    
    At least,
       1) mixing two transparents should be transparent and
       2) mixing two opaques should be opaque
---
 src/frontends/qt/ColorCache.cpp | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/frontends/qt/ColorCache.cpp b/src/frontends/qt/ColorCache.cpp
index 0949c2f3da..135c2f362a 100644
--- a/src/frontends/qt/ColorCache.cpp
+++ b/src/frontends/qt/ColorCache.cpp
@@ -132,16 +132,18 @@ ColorPair ColorCache::getAll(Color const & color, bool syscolors) const
 			        merge_colors.first.toRgb().green()) / 2,
 			       (base_colors.first.toRgb().blue() +
 			        merge_colors.first.toRgb().blue()) / 2,
-			       (base_colors.first.toRgb().alpha() +
-			        merge_colors.first.toRgb().alpha()) / 2),
+			       std::round(
+			           (1 - (1 - base_colors.first.toRgb().alphaF()) *
+			                (1 - merge_colors.first.toRgb().alphaF())) * 255)),
 			QColor((base_colors.second.toRgb().red() +
 			        merge_colors.second.toRgb().red()) / 2,
 			       (base_colors.second.toRgb().green() +
 			        merge_colors.second.toRgb().green()) / 2,
 			       (base_colors.second.toRgb().blue() +
 			        merge_colors.second.toRgb().blue()) / 2,
-			       (base_colors.second.toRgb().alpha() +
-			        merge_colors.second.toRgb().alpha()) / 2),
+			        std::round(
+			           (1 - (1 - base_colors.second.toRgb().alphaF()) *
+			                (1 - merge_colors.second.toRgb().alphaF())) * 255)),
 		};
 	}
 	// used by branches