T21a: Documents, editing

- markdown read with pulldown-cmark: marks for live styling, blocks, links, words
- forms::Highlight gains weight, slant and strike; measure and paint share runs
- Inter Italic embedded, so emphasis reads slanted
- forms::mention's popup and keys become Suggestions, shared with the editors
- RichTextEditor with FixedFormatToolbar, the floating toolbar, LinkEditor,
  @ and : suggestions, WordCount and ReadingTime
- MarkdownEditor: visual (a pressed block opens as source), source, split
- BlockEditor: prose, lists, to-dos, quotes, callouts, toggles, code, math,
  diagrams the app draws, dividers, tables that grow and merge, pictures to
  resize and align, cards for video and pages, columns, synced text; slash menu,
  markdown shortcuts, handles that drag and open a menu, one undo for all
- TodoItem and Checklist
- icons: heading-1..3, workflow, merge
- gallery: the Documents page with scripted states

src/documents/suggest.rs

@@ -0,0 +1,104 @@
+use std::rc::Rc;
+
+use gpui::{App, ElementId, Entity, SharedString, Window};
+
+use crate::forms::{
+    Choice, Pick, Suggestions, TextInput, active_trigger, emoji_found, handles_matching,
+};
+
+/// A trigger of the owner's: from the query and where it runs, the rows and what a pick does.
+pub(crate) type Offer<'a> = &'a dyn Fn(&str, usize, usize) -> (Vec<Choice>, Pick);
+
+/// An emoji name needs this many letters before `:` suggests, so a colon in prose stays quiet.
+const EMOJI_QUERY: usize = 2;
+
+/// Emoji shown for a `:` query.
+const EMOJI_SHOWN: usize = 6;
+
+/// An offer of nothing: no rows, so no pick can come.
+fn nothing() -> (Vec<Choice>, Pick) {
+    (
+        Vec::new(),
+        Rc::new(|_: usize, _: &mut Window, _: &mut App| unreachable!("an empty offer has no row")),
+    )
+}
+
+/// Replaces the trigger and its query, `at..caret`, with `text`, as one undo step.
+pub(crate) fn replace_trigger(
+    field: &Entity<TextInput>,
+    at: usize,
+    caret: usize,
+    text: &str,
+    cx: &mut App,
+) {
+    field.update(cx, |input, cx| {
+        input.select(at..caret, cx);
+        input.insert(text, cx);
+    });
+}
+
+/// What `@` and `:` offer at the field's caret: people by handle, emoji by name; `extra` adds a trigger of the owner's, such as `/` in blocks.
+pub(crate) fn offers(
+    id: ElementId,
+    field: &Entity<TextInput>,
+    people: &[SharedString],
+    extra: Option<(char, Offer)>,
+    cx: &App,
+) -> Suggestions {
+    let input = field.read(cx);
+    let (text, caret) = (input.text().to_string(), input.cursor());
+    let mut marks = vec!['@', ':'];
+    marks.extend(extra.as_ref().map(|(mark, _)| *mark));
+    let active = active_trigger(&text, caret, &marks);
+    let query = active.map_or("", |(at, mark)| &text[at + mark.len_utf8()..caret]);
+    let (rows, pick): (Vec<Choice>, Pick) = match active {
+        Some((at, '@')) => {
+            let found = handles_matching(people, query);
+            let rows = found
+                .iter()
+                .map(|name| Choice::new(name.clone(), format!("@{name}")))
+                .collect();
+            let field = field.clone();
+            (
+                rows,
+                Rc::new(move |pick: usize, _: &mut Window, cx: &mut App| {
+                    log::info!("mention: @{}", found[pick]);
+                    replace_trigger(&field, at, caret, &format!("@{} ", found[pick]), cx);
+                }),
+            )
+        }
+        Some((at, ':')) if query.len() >= EMOJI_QUERY => {
+            let found: Vec<&'static emojis::Emoji> = emoji_found(&query.to_lowercase())
+                .into_iter()
+                .take(EMOJI_SHOWN)
+                .collect();
+            let rows = found
+                .iter()
+                .map(|emoji| {
+                    let name = emoji.shortcode().unwrap_or(emoji.name());
+                    Choice::new(name, format!("{}  {name}", emoji.as_str()))
+                })
+                .collect();
+            let field = field.clone();
+            (
+                rows,
+                Rc::new(move |pick: usize, _: &mut Window, cx: &mut App| {
+                    log::info!("emoji: {}", found[pick].name());
+                    replace_trigger(&field, at, caret, found[pick].as_str(), cx);
+                }),
+            )
+        }
+        Some((at, mark)) => match &extra {
+            Some((extra_mark, build)) if *extra_mark == mark => build(query, at, caret),
+            _ => nothing(),
+        },
+        None => nothing(),
+    };
+    Suggestions {
+        id,
+        state: field.clone(),
+        trigger: active.map(|(at, _)| at),
+        rows,
+        pick,
+    }
+}