feat(ui): allow to set the highlight namespace per window

- reimplement 'winhl' in terms of highlight namespaces
- check for EOF in screen tests (to indicate a likely crash)

runtime/doc/api.txt

@@ -603,20 +603,6 @@ nvim__inspect_cell({grid}, {row}, {col})                *nvim__inspect_cell()*
     NB: if your UI doesn't use hlstate, this will not return hlstate first
     time.
 
-nvim__set_hl_ns({ns_id})                                   *nvim__set_hl_ns()*
-    Set active namespace for highlights.
-
-    NB: this function can be called from async contexts, but the semantics are
-    not yet well-defined. To start with |nvim_set_decoration_provider| on_win
-    and on_line callbacks are explicitly allowed to change the namespace
-    during a redraw cycle.
-
-    Attributes: ~
-        |api-fast|
-
-    Parameters: ~
-        {ns_id}  the namespace to activate
-
 nvim__stats()                                                  *nvim__stats()*
     Gets internal stats.
 
@@ -1414,6 +1400,26 @@ nvim_set_hl({ns_id}, {name}, {*val})                           *nvim_set_hl()*
                    set, cterm attributes will match those from the attribute
                    map documented above.
 
+nvim_set_hl_ns({ns_id})                                     *nvim_set_hl_ns()*
+    Set active namespace for highlights. This can be set for a single window,
+    see |nvim_win_set_hl_ns|.
+
+    Parameters: ~
+        {ns_id}  the namespace to use
+
+nvim_set_hl_ns_fast({ns_id})                           *nvim_set_hl_ns_fast()*
+    Set active namespace for highlights while redrawing.
+
+    This function meant to be called while redrawing, primarily from
+    |nvim_set_decoration_provider| on_win and on_line callbacks, which are
+    allowed to change the namespace during a redraw cycle.
+
+    Attributes: ~
+        |api-fast|
+
+    Parameters: ~
+        {ns_id}  the namespace to activate
+
 nvim_set_keymap({mode}, {lhs}, {rhs}, {*opts})             *nvim_set_keymap()*
     Sets a global |mapping| for the given mode.
 
@@ -2875,6 +2881,15 @@ nvim_win_set_height({window}, {height})                *nvim_win_set_height()*
         {window}  Window handle, or 0 for current window
         {height}  Height as a count of rows
 
+nvim_win_set_hl_ns({window}, {ns_id})                   *nvim_win_set_hl_ns()*
+    Set highlight namespace for a window. This will use highlights defined in
+    this namespace, but fall back to global highlights (ns=0) when missing.
+
+    This takes predecence over the 'winhighlight' option.
+
+    Parameters: ~
+        {ns_id}  the namespace to use
+
 nvim_win_set_var({window}, {name}, {value})               *nvim_win_set_var()*
     Sets a window-scoped (w:) variable
 

runtime/doc/options.txt

@@ -7096,10 +7096,12 @@ A jump table for the options with a short description can be found at |Q_op|.
 'winhighlight' 'winhl'	string (default empty)
 			local to window
 	Window-local highlights.  Comma-delimited list of highlight
-	|group-name| pairs "{hl-builtin}:{hl},..." where each {hl-builtin} is
-	a built-in |highlight-groups| item to be overridden by {hl} group in
-	the window.  Only built-in |highlight-groups| are supported, not
-	syntax highlighting (use |:ownsyntax| for that).
+	|group-name| pairs "{hl-from}:{hl-to},..." where each {hl-from} is
+	a |highlight-groups| item to be overridden by {hl-to} group in
+	the window.
+
+	Note: highlight namespaces take precedence over 'winhighlight'.
+	See |nvim_win_set_hl_ns| and |nvim_set_hl|.
 
 	Highlights of vertical separators are determined by the window to the
 	left of the separator.  The 'tabline' highlight of a tabpage is