Feature request: fullscreen TUI mode with mouse text selection/copy (like Claude Code)

What feature would you like to see?

Two related TUI features, both inspired by Claude Code's fullscreen mode:

1. Fullscreen TUI mode (alternate screen buffer), toggleable

Claude Code can run in a fullscreen mode where the TUI takes over the whole terminal. This gives a stable, app-controlled rendering surface: scrolling, layout, and mouse handling all behave consistently instead of interleaving with the host terminal's scrollback. A keybinding (e.g. ctrl+b) or a config option to enter/leave fullscreen would be ideal.

(Related: #2195 asks for the opposite direction — an option to disable the alternate screen for SSH scrollback. A user-facing toggle would serve both requests.)

2. Built-in mouse text selection that actually copies

In fullscreen mode, drag-selecting text with the mouse should create an app-level selection and copy it to the system clipboard — with an OSC 52 fallback when no native clipboard is available (SSH, tmux, browser code-server terminals).

Current pain point: in the inline TUI, mouse selection is unreliable. Drag-selecting shows a "Copied" hint, but the text never reaches the system clipboard — this is especially broken inside browser-based code-server terminals and tmux, where the terminal's own selection and the app's mouse handling fight each other. Users currently have to discover Shift-drag workarounds themselves.

(Related: #1174 proposes a conservative mouse policy that keeps native terminal selection and shows a Shift-drag hint. That helps, but an app-level selection+copy with OSC 52 fallback would make copy work out of the box in remote/browser terminals where native selection is unavailable entirely.)

Additional information

Environment: Kimi Code CLI on Linux, used daily inside tmux and browser code-server (web VS Code) terminals, often over SSH. In these environments neither the terminal's native selection nor naive OSC 52 passthrough works reliably, so app-managed selection with an OSC 52 fallback is the only option that works everywhere.

Start by tracing the existing TUI mouse-selection and clipboard handling, then review related issues #2195 and #1174 for compatibility constraints. Done should include a toggleable fullscreen mode and reliable app-level selection and copying, with OSC 52 fallback in SSH, tmux, and browser terminals.