Tracking/discussion issue for planned UI enhancements. NB: being mentioned on the list below is not a guarantee for being done at any specific time scale, nor is the order necessarily indicative of readiness/priority (except perhaps for the smaller follow-up items for multigrid, which I hope to get done pretty soon). Feel free to add stuff.

• multigrid ([RFC] Break up screen grid into resizable window grids #8455 merged)
  • improve size handling: non-screen functions should use requested size, not grid allocated size.
  • :term sizing issues (fix terminal_resize)
  • mouse support (API/UI: add nvim_input_mouse() including multigrid mouse support #9429)
  • messages on dedicated grid (when not ext_messages)
• floating windows ([RFC] Floating windows in TUI and Remote UI #6619)
  • for !ext_popupmenu: pum as float (Implement popupmenu as a floating grid internally to reduce flicker #9530)
  • for ext_popupmenu: add tests for grid anchoring
  • nvim_win_close to close window by id.
  • Ex-commands for floats (floating windows: Ex commands (:fclose, :fedit, ... or :float) #9663)
    • floating :pedit (Vim's :help preview-popup) #9920
  • pumblend but for floats (because why not)
• ext_messages ([RFC] ui: implement ext_messages to externalize all messages in msg area #7466)
• ext_windows: UI control of layout and wincmds (Externalize handling of window layout and control (wincmds) to UIs #8707)
• use ext_popupmenu events for wildmenu to support info and pum (UI: deprecate redundant ext_wildmenu events and allow TUI popupmenu for cmdline #9607)
• ext_statusline (I have some ideas/pre-WIP code...)
• external columns (number, signs) by transmitting wline_T info (no work done)
• "tabgrid" ("tabgrid" (multiple top-level windows) #7541 but with multigrid)
• transmit UI highlight definitions for ext_popupmenu using hl-Pmenu, etc. (highlight: expose builtin highlight groups using hl_group_set event #10504)
  • ensure that we always use concrete rgb colors for default_colors_set
  • allow subscribing non-default groups
• UI control of ext_popupmenu [RFC] API: select items in popupmenu #9445
  • For ext_cmdline maybe nvim_call_function('setcmdpos') already works (check + add docs/test)