content: Fix **bold code** rendering with regular weight

Our experience with variable-weight fonts is that, whenever we use
such a font, we need to specify a "wght" value, even if we want the
text to appear in the boring, regular weight. If we don't specify a
"wght", the text will appear in an extremely light weight.

We use a variable-weight font (Source Code Pro) for rendering inline
code spans, so that's why we were using weightVariableTextStyle
here. As an unfortunate result, when a code span appears inside a
bold ("strong") span, the outer span's bold-weight style has been
getting clobbered, and the code span appears in regular weight.

Fortunately, removing the `weightVariableTextStyle` from the inline
code style doesn't actually cause any inline spans to appear at
minimum weight. This is a relief, and it follows from the presence
of a `weightVariableTextStyle` applied at the paragraph-content
level. We use `weightVariableTextStyle` there because the paragraph
font, Source Sans 3, is also a variable-weight font. Moreover,
Source Code Pro and Source Sans 3 seem closely related (and were
designed by the same person), so it's likely that their "wght" axes
are identical.

Fixes: zulip#498

Author: chrisbobbe

lib/widgets/content.dart

@@ -600,9 +600,18 @@ class _InlineContentBuilder {
 final _kInlineMathStyle = _kInlineCodeStyle.merge(TextStyle(
   backgroundColor: const HSLColor.fromAHSL(1, 240, 0.4, 0.93).toColor()));
 
+// Even though [kMonospaceTextStyle] is a variable-weight font,
+// it's acceptable to skip [weightVariableTextStyle] here,
+// assuming the text gets the effect of [weightVariableTextStyle]
+// through inheritance, e.g., from a [DefaultTextStyle].
 final _kInlineCodeStyle = kMonospaceTextStyle
   .merge(const TextStyle(
     backgroundColor: Color(0xffeeeeee),
+    fontSize: 0.825 * kBaseFontSize));
+
+final _kCodeBlockStyle = kMonospaceTextStyle
+  .merge(const TextStyle(
+    backgroundColor: Color.fromRGBO(255, 255, 255, 1),
     fontSize: 0.825 * kBaseFontSize))
   .merge(
     // TODO(a11y) pass a BuildContext, to handle platform request for bold text.
@@ -612,14 +621,6 @@ final _kInlineCodeStyle = kMonospaceTextStyle
     //   frequently. Then consumers can just look it up on the InheritedWidget.
     weightVariableTextStyle(null));
 
-final _kCodeBlockStyle = kMonospaceTextStyle
-  .merge(const TextStyle(
-    backgroundColor: Color.fromRGBO(255, 255, 255, 1),
-    fontSize: 0.825 * kBaseFontSize))
-  .merge(
-    // TODO(a11y) pass a BuildContext; see comment in _kInlineCodeStyle above.
-    weightVariableTextStyle(null));
-
 // const _kInlineCodeLeftBracket = '⸤';
 // const _kInlineCodeRightBracket = '⸣';
 // Some alternatives: