fix(highlight): floating windows inherit NormalFloat from global-ns

Problem:
floating windows did not correctly inherit the NormalFloat highlight
group from the global namespace when it was not defined in the window-specific
namespace. This led to floating windows losing their background highlight when
switching between namespaces.

Solution:
Updated the window highlight logic in update_window_hl() to handle the fallback.

This fix resolves issues with floating window backgrounds not displaying as expected
in certain namespace configurations.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/nvim/highlight.c b/src/nvim/highlight.c
index 8729c74ce8..b802eab4b9 100644
--- a/src/nvim/highlight.c
+++ b/src/nvim/highlight.c
@@ -370,12 +370,15 @@ void update_window_hl(win_T *wp, bool invalid)
 
   // determine window specific background set in 'winhighlight'
   bool float_win = wp->w_floating && !wp->w_config.external;
-  if (float_win && hl_def[HLF_NFLOAT] != 0) {
+  if (float_win && hl_def[HLF_NFLOAT] != 0 && ns_id > 0) {
     wp->w_hl_attr_normal = hl_def[HLF_NFLOAT];
   } else if (hl_def[HLF_COUNT] > 0) {
     wp->w_hl_attr_normal = hl_def[HLF_COUNT];
+  } else if (float_win) {
+    wp->w_hl_attr_normal = HL_ATTR(HLF_NFLOAT) > 0
+                           ? HL_ATTR(HLF_NFLOAT) : highlight_attr[HLF_NFLOAT];
   } else {
-    wp->w_hl_attr_normal = float_win ? HL_ATTR(HLF_NFLOAT) : 0;
+    wp->w_hl_attr_normal = 0;
   }
 
   if (wp->w_floating) {