feat(health): detect tmux RGB support via `client_termfeatures`

Problem: On tmux v3.2+, the `terminal-features` option may be used to enable RGB
capabilities over `terminal-overrides`. However, `show-messages` cannot be used
to detect if RGB capabilities are enabled using `terminal-features`.

Solution: Try to use `display-message -p #{client_termfeatures}` instead.
The returned features include "RGB" if either "RGB" is set in
`terminal-features`, or if "Tc" or "RGB" is set in `terminal-overrides` (as
before).
Nothing is returned by tmux versions older than v3.2, so fallback to checking
`show-messages` in that case.

Also, un-Vimscriptify the previous logic a bit, and change the error message to
point to using the `terminal-features` option instead for newer tmux versions.

1 files changed, 18 insertions, 11 deletions

diff --git a/runtime/lua/nvim/health.lua b/runtime/lua/nvim/health.lua
index 02e08bd381..b76106f241 100644
--- a/runtime/lua/nvim/health.lua
+++ b/runtime/lua/nvim/health.lua
@@ -326,17 +326,24 @@ local function check_tmux()
   end
 
   -- check for RGB capabilities
-  local info = vim.fn.system({ 'tmux', 'show-messages', '-JT' })
-  local has_tc = vim.fn.stridx(info, ' Tc: (flag) true') ~= -1
-  local has_rgb = vim.fn.stridx(info, ' RGB: (flag) true') ~= -1
-  if not has_tc and not has_rgb then
-    health.report_warn(
-      "Neither Tc nor RGB capability set. True colors are disabled. |'termguicolors'| won't work properly.",
-      {
-        "Put this in your ~/.tmux.conf and replace XXX by your $TERM outside of tmux:\nset-option -sa terminal-overrides ',XXX:RGB'",
-        "For older tmux versions use this instead:\nset-option -ga terminal-overrides ',XXX:Tc'",
-      }
-    )
+  local info = vim.fn.system({ 'tmux', 'display-message', '-p', '#{client_termfeatures}' })
+  info = vim.split(vim.trim(info), ',', { trimempty = true })
+  if not vim.tbl_contains(info, 'RGB') then
+    local has_rgb = false
+    if #info == 0 then
+      -- client_termfeatures may not be supported; fallback to checking show-messages
+      info = vim.fn.system({ 'tmux', 'show-messages', '-JT' })
+      has_rgb = info:find(' Tc: (flag) true', 1, true) or info:find(' RGB: (flag) true', 1, true)
+    end
+    if not has_rgb then
+      health.report_warn(
+        "Neither Tc nor RGB capability set. True colors are disabled. |'termguicolors'| won't work properly.",
+        {
+          "Put this in your ~/.tmux.conf and replace XXX by your $TERM outside of tmux:\nset-option -sa terminal-features ',XXX:RGB'",
+          "For older tmux versions use this instead:\nset-option -ga terminal-overrides ',XXX:Tc'",
+        }
+      )
+    end
   end
 end