T21b and T21c round 1 fixes

- a page centers only while it fits the view; a wider one starts in reach
- the reveal runs after the pages' prepaint and keys on the hit itself, so a
  new hit at the same index and a hit current from the first frame come into view
- the chapter list scrolls
- zen takes focus when turned on from outside, so Escape leaves it, and hands
  focus back
- the gallery keeps a page's due date and tags

src/documents/reading.rs

@@ -1,8 +1,9 @@
 use std::{collections::BTreeMap, rc::Rc};
 
 use gpui::{
-    AnyElement, App, Bounds, ElementId, FontWeight, InteractiveElement, IntoElement, ParentElement,
-    RenderOnce, ScrollHandle, SharedString, Styled, Window, canvas, div, fill, prelude::*, size,
+    AnyElement, App, Bounds, ElementId, FocusHandle, FontWeight, InteractiveElement, IntoElement,
+    ParentElement, RenderOnce, ScrollHandle, SharedString, Styled, Window, canvas, div, fill,
+    prelude::*, size,
 };
 use smallvec::SmallVec;
 
@@ -271,12 +272,35 @@ impl ParentElement for ZenMode {
     }
 }
 
+/// Zen's own focus, where focus was before zen took it, and whether zen was on last frame.
+struct Quiet {
+    focus: FocusHandle,
+    previous: Option<FocusHandle>,
+    was_on: bool,
+}
+
 impl RenderOnce for ZenMode {
-    fn render(self, _: &mut Window, cx: &mut App) -> impl IntoElement {
-        let theme = cx.theme();
+    fn render(self, window: &mut Window, cx: &mut App) -> impl IntoElement {
         let (on, exit) = (self.on, self.on_exit);
+        let quiet = window.use_keyed_state((self.id.clone(), "quiet"), cx, |_, cx| Quiet {
+            focus: cx.focus_handle(),
+            previous: None,
+            was_on: false,
+        });
+        let (focus, was_on) = (quiet.read(cx).focus.clone(), quiet.read(cx).was_on);
+        if on && !was_on && !focus.contains_focused(window, cx) {
+            log::info!("zen mode: takes focus, so Escape reaches it");
+            let previous = window.focused(cx);
+            window.focus(&focus);
+            quiet.update(cx, |quiet, _| quiet.previous = previous);
+        }
+        if on != was_on {
+            quiet.update(cx, |quiet, _| quiet.was_on = on);
+        }
+        let theme = cx.theme();
         div()
             .id(self.id.clone())
+            .track_focus(&focus)
             .flex()
             .flex_col()
             .on_key_down(move |event, window, cx| {
@@ -286,6 +310,9 @@ impl RenderOnce for ZenMode {
                 {
                     cx.stop_propagation();
                     log::info!("zen mode: leaves");
+                    if let Some(previous) = quiet.update(cx, |quiet, _| quiet.previous.take()) {
+                        window.focus(&previous);
+                    }
                     exit(window, cx);
                 }
             })