fix(material/radio): rendering artifacts at some zoom levels

Currently the circle inside the radio button is rendered out as an element with a solid border. Presumably MDC did it this way, because it allows more CSS rules to be combined and it handles high contrast mode automatically. The problem with this approach is that depending on the user's screen and zoom level, there can be rendering artifacts on Windows like a dot in the middle of the radio button.

These changes resolve the rendering artifacts by using a background color for the circle and which is overridden to `CanvasText` for high contrast mode.

Fixes #31466.

Author: crisbeto

src/material/radio/BUILD.bazel

@@ -56,6 +56,7 @@ sass_library(
     srcs = ["_radio-common.scss"],
     deps = [
         ":m2",
+        "//src/cdk:sass_lib",
         "//src/material/core/tokens:token_utils",
     ],
 )

src/material/radio/_radio-common.scss

@@ -1,3 +1,4 @@
+@use '@angular/cdk';
 @use './m3-radio';
 @use '../core/tokens/token-utils';
 
@@ -47,9 +48,14 @@ $fallbacks: m3-radio.get-tokens();
       }
 
       &:hover > .mdc-radio__native-control:enabled:checked + .mdc-radio__background {
-        > .mdc-radio__outer-circle,
+        $token: 'radio-selected-hover-icon-color';
+
+        > .mdc-radio__outer-circle {
+          border-color: token-utils.slot($token, $fallbacks);
+        }
+
         > .mdc-radio__inner-circle {
-          border-color: token-utils.slot(radio-selected-hover-icon-color, $fallbacks);
+          background-color: token-utils.slot($token, $fallbacks, currentColor);
         }
       }
 
@@ -60,9 +66,14 @@ $fallbacks: m3-radio.get-tokens();
       }
 
       &:active > .mdc-radio__native-control:enabled:checked + .mdc-radio__background {
-        > .mdc-radio__outer-circle,
+        $token: 'radio-selected-pressed-icon-color';
+
+        > .mdc-radio__outer-circle {
+          border-color: token-utils.slot($token, $fallbacks);
+        }
+
         > .mdc-radio__inner-circle {
-          border-color: token-utils.slot(radio-selected-pressed-icon-color, $fallbacks);
+          background-color: token-utils.slot($token, $fallbacks, currentColor);
         }
       }
     }
@@ -113,11 +124,16 @@ $fallbacks: m3-radio.get-tokens();
     box-sizing: border-box;
     width: 100%;
     height: 100%;
-    transform: scale(0, 0);
-    border-width: 10px;
-    border-style: solid;
+    transform: scale(0);
     border-radius: 50%;
-    transition: _exit-transition(transform), _exit-transition(border-color);
+    transition: _exit-transition(transform), _exit-transition(background-color);
+
+    @include cdk.high-contrast {
+      // Override the color, because solid colors don't show up by default in
+      // high contrast mode. We need !important here, because the various state
+      // selectors are really specific and duplicating them will be brittle.
+      background-color: CanvasText !important;
+    }
   }
 
   .mdc-radio__native-control {
@@ -142,7 +158,7 @@ $fallbacks: m3-radio.get-tokens();
         }
 
         > .mdc-radio__inner-circle {
-          transition: _enter-transition(transform), _enter-transition(border-color);
+          transition: _enter-transition(transform), _enter-transition(background-color);
         }
       }
     }
@@ -162,12 +178,18 @@ $fallbacks: m3-radio.get-tokens();
       }
 
       + .mdc-radio__background {
+        $color-token: 'radio-disabled-selected-icon-color';
+        $opacity-token: token-utils.slot(radio-disabled-selected-icon-opacity, $fallbacks);
         cursor: default;
 
-        > .mdc-radio__inner-circle,
         > .mdc-radio__outer-circle {
-          border-color: token-utils.slot(radio-disabled-selected-icon-color, $fallbacks);
-          opacity: token-utils.slot(radio-disabled-selected-icon-opacity, $fallbacks);
+          border-color: token-utils.slot($color-token, $fallbacks);
+          opacity: $opacity-token;
+        }
+
+        > .mdc-radio__inner-circle {
+          background-color: token-utils.slot($color-token, $fallbacks, currentColor);
+          opacity: $opacity-token;
         }
       }
     }
@@ -178,25 +200,35 @@ $fallbacks: m3-radio.get-tokens();
       }
 
       &:checked + .mdc-radio__background {
-        > .mdc-radio__outer-circle,
+        $token: 'radio-selected-icon-color';
+
+        > .mdc-radio__outer-circle {
+          border-color: token-utils.slot($token, $fallbacks);
+        }
+
         > .mdc-radio__inner-circle {
-          border-color: token-utils.slot(radio-selected-icon-color, $fallbacks);
+          background-color: token-utils.slot($token, $fallbacks, currentColor);
         }
       }
 
       @if ($is-interactive) {
         &:focus:checked + .mdc-radio__background {
-          > .mdc-radio__inner-circle,
+          $token: 'radio-selected-focus-icon-color';
+
           > .mdc-radio__outer-circle {
-            border-color: token-utils.slot(radio-selected-focus-icon-color, $fallbacks);
+            border-color: token-utils.slot($token, $fallbacks);
+          }
+
+          > .mdc-radio__inner-circle {
+            background-color: token-utils.slot($token, $fallbacks, currentColor);
           }
         }
       }
     }
 
     &:checked + .mdc-radio__background > .mdc-radio__inner-circle {
       transform: scale(0.5);
-      transition: _enter-transition(transform), _enter-transition(border-color);
+      transition: _enter-transition(transform), _enter-transition(background-color);
     }
   }
 
@@ -215,10 +247,17 @@ $fallbacks: m3-radio.get-tokens();
       &:hover .mdc-radio__native-control:checked + .mdc-radio__background,
       .mdc-radio__native-control:checked:focus + .mdc-radio__background,
       .mdc-radio__native-control + .mdc-radio__background {
-        > .mdc-radio__inner-circle,
+        $color-token: 'radio-disabled-selected-icon-color';
+        $opacity-token: token-utils.slot(radio-disabled-selected-icon-opacity, $fallbacks);
+
         > .mdc-radio__outer-circle {
-          border-color: token-utils.slot(radio-disabled-selected-icon-color, $fallbacks);
-          opacity: token-utils.slot(radio-disabled-selected-icon-opacity, $fallbacks);
+          border-color: token-utils.slot($color-token, $fallbacks);
+          opacity: $opacity-token;
+        }
+
+        > .mdc-radio__inner-circle {
+          background-color: token-utils.slot($color-token, $fallbacks, currentColor);
+          opacity: $opacity-token;
         }
       }
     }