katex [nfc]: Consolidate where we cancel out ambient text styles

In the inline-math case, we're about to pass the actual ambient text
style, which may have italic, strikethrough, bold, or link
formatting. Consolidate the code that overrides each of those, so
that it reads as intentional, and add TODOs for overrides that we
might not want to keep.

Author: chrisbobbe

lib/widgets/katex.dart

@@ -10,12 +10,35 @@ import 'content.dart';
 
 /// Creates a base text style for rendering KaTeX content.
 ///
-/// This applies the CSS styles defined in .katex class in katex.scss :
+/// This cancels out some attributes that may be ambient from Zulip content
+/// (italic, bold, etc.)
+/// and applies the CSS styles defined in .katex class in katex.scss :
 ///   https://github.com/KaTeX/KaTeX/blob/613c3da8/src/styles/katex.scss#L13-L15
 ///
 /// Requires the [style.fontSize] to be non-null.
-TextStyle mkBaseKatexTextStyle(TextStyle style) {
+TextStyle mkBaseKatexTextStyle(TextStyle style, Color baseColor) {
   return style.copyWith(
+    ////// Overrides of our own styles:
+
+    // Bold formatting is removed below by setting FontWeight.normal…
+    // Just for completeness, remove "wght", but it wouldn't do anything anyway
+    // since KaTeX_Main is not a variable-weight font.
+    fontVariations: [],
+    // Remove link color, but
+    // TODO(#1823) do we want to do that? Web doesn't.
+    color: baseColor,
+    // Italic is removed below.
+
+    // Strikethrough is removed below, which affects formatting of rendered
+    // KatexSpanNodes…but a single strikethrough on the whole KatexWidget will
+    // be visible as long as it doesn't paint an opaque background. (The line
+    // "shows through" from an ancestor span.) I think we're happy with this:
+    // the message author asked for a strikethrough by wrapping the math in ~~,
+    // but we should render it as one unbroken line, not separate lines on each
+    // KaTeX span.
+
+    ////// From the .katex class in katex.scss:
+
     fontSize: style.fontSize! * 1.21,
     fontFamily: 'KaTeX_Main',
     height: 1.2,
@@ -45,8 +68,8 @@ class KatexWidget extends StatelessWidget {
     return Directionality(
       textDirection: TextDirection.ltr,
       child: DefaultTextStyle(
-        style: mkBaseKatexTextStyle(textStyle).copyWith(
-          color: ContentTheme.of(context).textStylePlainParagraph.color),
+        style: mkBaseKatexTextStyle(textStyle,
+          ContentTheme.of(context).textStylePlainParagraph.color!),
         child: widget));
   }
 }