T24d round 2 fix

- BrowserPreview keeps a ScrollHandle for its strip and a tracked focus
  handle per thumbnail; the focused thumbnail scrolls into view once, through
  forms::options::reveal and the canvas now shared as revealer
- Popup draws its reveal through the same revealer
- test tab_brings_a_hidden_frame_into_view

src/forms/mod.rs

@@ -83,7 +83,7 @@ pub use multi::MultiSelect;
 pub use number::{NumberInput, ScrubInput};
 pub use options::Choice;
 pub(crate) use options::{
-    OnFlag, OnValue, OnValues, Pick, Run, float, float_height, step, surface,
+    OnFlag, OnValue, OnValues, Pick, Run, float, float_height, reveal, revealer, step, surface,
 };
 pub use pairs::{FieldArray, KeyValueInput, ListInput};
 pub use path::PathInput;