[idea] vim/tmux-style H / M / L cursor movement in copy mode

[masutaka]

idea / problem

Copy mode now supports Ctrl-f / Ctrl-b page navigation (added in #885, released in v0.7.2), plus g/G, Ctrl-u/Ctrl-d, and h/j/k/l motions. Thanks for that!

However, the vim/tmux copy-mode motions H / M / L are not available:

• H → move cursor to the top line of the visible screen (High)
• M → move cursor to the middle line of the visible screen (Middle)
• L → move cursor to the bottom line of the visible screen (Low)

For users coming from vim or tmux copy-mode-vi, these are a very common way to reposition the cursor within the current viewport without scrolling. Their absence makes copy-mode navigation feel slightly incomplete compared to the h/j/k/l and Ctrl-f/Ctrl-b motions that already exist.

Note this is different from #731 (z to center the cursor line on screen): H/M/L move the cursor to a fixed position within the current viewport, they do not scroll the view.

requested change

Add the following key bindings in copy mode, matching vim/tmux copy-mode-vi:

• H → move cursor to the top visible line
• M → move cursor to the middle visible line
• L → move cursor to the bottom visible line

Looking at src/app/input/copy_mode.rs, copy_mode_command_char() already resolves Shift into uppercase chars (so H/M/L reach the match ch block), but there are currently no arms for them. This looks implementable by reusing the existing viewport/cursor movement logic (the same machinery used by g/G and the page-scroll helpers), without changing any existing behavior.

why you want this

This improves vim/tmux compatibility and makes copy-mode cursor positioning more intuitive for terminal users who already rely on H/M/L. The change is additive and does not alter any existing keybinding.