Project · focus that follows: segments, the tracker's action, tests

A segmented control too narrow for its segments now scrolls sideways
in a strip of its own, and a segment that takes focus comes into view:
its focus is a tab_stop handle, and layout::reveal_when_focused, now
shared with the board's cards, measures it from a box that holds its
ring. The DatabaseView's own strip goes.

The time tracker's Start and Stop are one Button that keeps one handle
through the new Button::focus_handle. A start moves focus to it, a stop
hands focus back to the field, and the field starts on Enter's release,
so the release no longer clicks the Stop that took focus.

ProgressRing's percent refuses content inside, as content refuses a
percent. Tests: Enter starts, Enter stops after 90 seconds, and the
field takes the keys again; Skip moves on at once; a focused Timeline
segment at 280px comes into view; the ring's refusal.

AGENTS.md

@@ -76,6 +76,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - gpui draws a window's first frame inside `open_window`. State the first frame reads, such as `cx.set_menus`, is set before it opens.
 - gpui clicks a focused element when Enter or Space is released. An overlay that hands focus back picks on release too; a pick on press returns focus first, and the release clicks the opener again. An edit in place that hands focus back keeps its text on Enter's release (`forms::InlineEdit`). The press itself bubbles, so a container that takes Space leaves a focused button its own: `overlays::media_button` stops it, and a button in a `SelectableList` row stops Space and Enter at its box (`mail::MailItem`'s star, `messaging::PinnedMessages`' Unpin).
 - A focus handle a component hands back lives across all its modes: take it with `tab_stop` before any early return, or keyed state drops it while the other mode draws and the handed-back focus lands nowhere. A thread's message keeps one handle folded and open, a Tab stop only while folded (`mail::MailThreadView`).
+- A button whose words and action change with a mode keeps one handle through `Button::focus_handle`, and the component moves focus to it or back to its field as the mode turns (`project::TimeTracker`).
 - A panel that opens from something other than a button goes through `overlays::Popover::with_opener`, which hands the opener its toggle; focus, placement and every way out stay in the popover (`calendar::EventPopover`'s chip).
 - A dialog goes through `overlays::Dialog`: scrim, focus trap, Escape and focus return live there. Its close runs on every way out, so a dialog with two answers marks the chosen one before it closes (`messaging::IncomingCallDialog`). AlertDialog, ConfirmDialog and PromptDialog build on it; a fullscreen dialog fills the window below its title bar. A dialog's card fits the window, a title bar's height clear at each end; its body scrolls and its actions stay.
 - Controls on a dark layer over pictures are `overlays::media_button`, light icons that dim when they have nowhere to go, for the lightbox and a video player's bar alike.
@@ -94,7 +95,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - Only gpui can build `ExternalPaths` with paths, so drop rules live in `forms::files::dropped`, where tests reach them. `taken` adds file kinds by extension, for drops and the dialog alike (`DropZone::kinds`).
 - A stroke drawn with the pointer goes through `forms::pen`: a press starts it, a drag extends it, and the lift hands it over, in or out of the pad (`SignaturePad`, `generative::InpaintCanvas`, `calendar::TimeGrid`'s drag on empty time).
 - gpui runs key bindings before key listeners. A container takes a child's bound key through the action with `capture_action`, as `Form` takes `Submit`.
-- `ScrollHandle::scroll_to_item` runs in the container's prepaint against last frame's bounds and overflow, so a call before a list's first frame does nothing. Call it from a child's prepaint once the handle has bounds, then request one frame (`navigation::editor_tabs`). A floating list reveals its cursor, and a strip its focused item, through `forms::options::reveal` and `revealer`. A focused Tab stop deeper in a scroll box sets the box's offset whole from its own painted bounds in prepaint, once per focus, through `layout::bring_into_view` along the box's axis; the canvas measures the stop with its ring (`messaging::GifPicker`). Never poll layout with `request_animation_frame`.
+- `ScrollHandle::scroll_to_item` runs in the container's prepaint against last frame's bounds and overflow, so a call before a list's first frame does nothing. Call it from a child's prepaint once the handle has bounds, then request one frame (`navigation::editor_tabs`). A floating list reveals its cursor, and a strip its focused item, through `forms::options::reveal` and `revealer`. A focused Tab stop deeper in a scroll box sets the box's offset whole from its own painted bounds in prepaint, once per focus, through `layout::bring_into_view` along the box's axis; the canvas measures the stop with its ring (`messaging::GifPicker`). `layout::reveal_when_focused` is that canvas; it lies over a box that holds the stop's ring (`buttons::SegmentedControl`, `project::KanbanBoard`). Never poll layout with `request_animation_frame`.
 - gpui animations run on the wall clock; timers run on the executor's clock, which tests advance by hand. A test that needs an animation settled turns on the theme's reduced motion. Clocks read `background_executor().now()` and wake only their own view.
 - Machine-read marks, QR codes and barcodes, paint `ink` on `paper`: dark on light in both themes, so cameras read them.
 - A layer over pressable content that must still pass scrolls to its ancestors uses `block_mouse_except_scroll`; `occlude` hides them from scrolls too.