Context

The call bar is a see-through window sized to the bar plus room for what opens beside it: notes and asks, menus, and the tooltips of a standing bar (0047). Every pixel of that window took clicks, so a desktop icon or a window next to the bar couldn't be clicked, and picking a window to hide missed clicks under it. A standing bar grew its window while the pointer was on it, for its tooltips, which moved the bar slightly.

Decision

The page says where it can be clicked. It sends the shell the boxes of the bar (or its pill), the notes and asks beside it, and whether a button is held (pair_bar_hit).
The shell follows the pointer. While the bar is open, a thread reads the pointer's position about 60 times a second and makes the window ignore the pointer (set_ignore_cursor_events) when it's more than 8 px from every box. While a menu is open, a button is held or the bar is dragged, the whole window takes clicks, so a click beside a menu closes it and a drag never loses its release.
A standing bar keeps its room. Its window always has the room beside it, where clicks pass through, so pointing at the bar never resizes the window.

Consequences

What's beside the bar can be clicked, on macOS and Windows alike.
Before the page has said where it can be clicked, or when the pointer's position can't be read, the whole window takes clicks, as before.
Hover effects on the bar start a frame after the pointer reaches it, since the window ignored the pointer until then.
One thread runs for the bar's life, waking every 16 ms.