markdown: Make push_div work with Stateful<Div>s (#22622)

This PR updates the `push_div` method in the `MarkdownElementBuilder` to
support taking in a `Stateful<Div>`.

This is some groundwork for supporting horizontal scrolling in Markdown
code blocks.

Release Notes:

- N/A

crates/markdown/src/markdown.rs

@@ -6,8 +6,8 @@ use gpui::{
     actions, point, quad, AnyElement, AppContext, Bounds, ClipboardItem, CursorStyle,
     DispatchPhase, Edges, FocusHandle, FocusableView, FontStyle, FontWeight, GlobalElementId,
     Hitbox, Hsla, KeyContext, Length, MouseDownEvent, MouseEvent, MouseMoveEvent, MouseUpEvent,
-    Point, Render, StrikethroughStyle, StyleRefinement, StyledText, Task, TextLayout, TextRun,
+    Point, Render, Stateful, StrikethroughStyle, StyleRefinement, StyledText, Task, TextLayout,
-    TextStyle, TextStyleRefinement, View,
+    TextRun, TextStyle, TextStyleRefinement, View,
 };
 use language::{Language, LanguageRegistry, Rope};
 use parser::{parse_links_only, parse_markdown, MarkdownEvent, MarkdownTag, MarkdownTagEnd};
@@ -785,8 +785,52 @@ impl IntoElement for MarkdownElement {
     }
 }
 
+enum AnyDiv {
+    Div(Div),
+    Stateful(Stateful<Div>),
+}
+
+impl AnyDiv {
+    fn into_any_element(self) -> AnyElement {
+        match self {
+            Self::Div(div) => div.into_any_element(),
+            Self::Stateful(div) => div.into_any_element(),
+        }
+    }
+}
+
+impl From<Div> for AnyDiv {
+    fn from(value: Div) -> Self {
+        Self::Div(value)
+    }
+}
+
+impl From<Stateful<Div>> for AnyDiv {
+    fn from(value: Stateful<Div>) -> Self {
+        Self::Stateful(value)
+    }
+}
+
+impl Styled for AnyDiv {
+    fn style(&mut self) -> &mut StyleRefinement {
+        match self {
+            Self::Div(div) => div.style(),
+            Self::Stateful(div) => div.style(),
+        }
+    }
+}
+
+impl ParentElement for AnyDiv {
+    fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
+        match self {
+            Self::Div(div) => div.extend(elements),
+            Self::Stateful(div) => div.extend(elements),
+        }
+    }
+}
+
 struct MarkdownElementBuilder {
-    div_stack: Vec<Div>,
+    div_stack: Vec<AnyDiv>,
     rendered_lines: Vec<RenderedLine>,
     pending_line: PendingLine,
     rendered_links: Vec<RenderedLink>,
@@ -812,7 +856,7 @@ struct ListStackEntry {
 impl MarkdownElementBuilder {
     fn new(base_text_style: TextStyle, syntax_theme: Arc<SyntaxTheme>) -> Self {
         Self {
-            div_stack: vec![div().debug_selector(|| "inner".into())],
+            div_stack: vec![div().debug_selector(|| "inner".into()).into()],
             rendered_lines: Vec::new(),
             pending_line: PendingLine::default(),
             rendered_links: Vec::new(),
@@ -841,11 +885,12 @@ impl MarkdownElementBuilder {
         self.text_style_stack.pop();
     }
 
-    fn push_div(&mut self, mut div: Div, range: &Range<usize>, markdown_end: usize) {
+    fn push_div(&mut self, div: impl Into<AnyDiv>, range: &Range<usize>, markdown_end: usize) {
+        let mut div = div.into();
         self.flush_text();
 
         if range.start == 0 {
-            //first element, remove top margin
+            // Remove the top margin on the first element.
             div.style().refine(&StyleRefinement {
                 margin: gpui::EdgesRefinement {
                     top: Some(Length::Definite(px(0.).into())),
@@ -856,6 +901,7 @@ impl MarkdownElementBuilder {
                 ..Default::default()
             });
         }
+
         if range.end == markdown_end {
             div.style().refine(&StyleRefinement {
                 margin: gpui::EdgesRefinement {
@@ -867,12 +913,13 @@ impl MarkdownElementBuilder {
                 ..Default::default()
             });
         }
+
         self.div_stack.push(div);
     }
 
     fn pop_div(&mut self) {
         self.flush_text();
-        let div = self.div_stack.pop().unwrap().into_any();
+        let div = self.div_stack.pop().unwrap().into_any_element();
         self.div_stack.last_mut().unwrap().extend(iter::once(div));
     }
 
@@ -973,7 +1020,7 @@ impl MarkdownElementBuilder {
         debug_assert_eq!(self.div_stack.len(), 1);
         self.flush_text();
         RenderedMarkdown {
-            element: self.div_stack.pop().unwrap().into_any(),
+            element: self.div_stack.pop().unwrap().into_any_element(),
             text: RenderedText {
                 lines: self.rendered_lines.into(),
                 links: self.rendered_links.into(),