fix(ui2): mouse movement dismisses expanded messages #41732

Problem:
cmd_on_key() tests the key it was handed against the literal string
'<MouseMove>', but at that point "typed" still holds raw key bytes. Thus
the comparison never holds and the branch its own comment describes
is dead. Moving the mouse over an expanded cmdline collapses it.

Solution:
Translate the key once, up front, so every comparison in the function
sees the same form.

AI-assisted

runtime/lua/vim/_core/ui2/messages.lua

@@ -598,6 +598,7 @@ end
 
 local typed_g = false
 local function cmd_on_key(key, typed)
+  typed = fn.keytrans(typed)
   -- Don't dismiss for non-typed keys and mouse movement. When 'g' is passed (typed
   -- or mapped), wait until the next key to avoid flickering when the pager is opened.
   if not typed_g and (typed == '' or (typed == '<MouseMove>' or typed == 'g' or key == 'g')) then
@@ -611,7 +612,6 @@ local function cmd_on_key(key, typed)
     return
   end
   vim.on_key(nil, ui.ns)
-  typed = fn.keytrans(typed)
 
   -- Check if window was entered and reopen with original config. A shown (but not entered)
   -- pager is dismissed instead; "g<" passes through to reopen and enter it.

test/functional/ui/messages2_spec.lua

@@ -544,6 +544,8 @@ describe('messages2', function()
       baz                                                  |
       {16::}{15:echo} {26:"baz"}                                          |
     ]])
+    -- Moving the mouse does not dismiss the pager.
+    api.nvim_input_mouse('move', '', '', 0, 0, 3)
     -- Subsequent typed commands are appended to the pager.
     feed(':echo "typed append"<CR>')
     screen:expect([[