Mouse Back/Forward follow the last-used panel, including right-panel tab history #22519

Problem

I use the mouse Back and Forward buttons a lot. Today they always move the app's route between threads, unless the pointer is directly over a browser page. Two cases don't match what I expect:

1. After working in the preview panel, for example clicking into a Browser tab whose Back/Forward arrows are enabled, then moving the pointer away from the page, Back switches threads instead of going back in the preview.
2. Switching between right-panel tabs (two terminals, a browser and a file, and so on) creates no history. After clicking from terminal A to terminal B, Back doesn't return to terminal A.

The pointer-over-page case for streamed previews is a separate bug: #22516.

Proposed direction

Make Back/Forward follow the panel I last interacted with:

• If the chat or sidebar was used last, keep today's thread navigation.
• If the right panel was used last, Back/Forward step through that thread's right-panel history. That covers the active browser tab's page history first, and then tab selections in the order I made them.

Questions for maintainers

• Is "last-interacted panel" the right signal, rather than pointer location or keyboard focus?
• Should browser page history and tab-selection history be one stack, or should Back only switch tabs once the browser's own history is exhausted?
• Should this also apply to the existing back/forward keybindings, or to mouse buttons only?
• Is there a smaller first step you'd prefer, such as tab-selection history alone?

Related but different: #12637 (reopening closed tabs), and #7470 / #15035 (cycling through tabs in order).

I'd like explicit approval of the direction and scope before preparing a PR.