feat(ui): don't show unfocusable windows in :tabs, 'tabline' #27984

Problem:  Floating windows with focusable set to false can reasonably be
          expected to be UI elements but are listed in some outputs that
          should contain only regular windows.
Solution: Hide unfocusable floating windows from the default tabline and
          :tabs.

src/nvim/api/win_config.c

@@ -130,7 +130,7 @@
 ///   - focusable: Enable focus by user actions (wincmds, mouse events).
 ///       Defaults to true. Non-focusable windows can be entered by
 ///       |nvim_set_current_win()|, or, when the `mouse` field is set to true,
-///       by mouse events.
+///       by mouse events. See |focusable|.
 ///   - mouse: Specify how this window interacts with mouse events.
 ///       Defaults to `focusable` value.
 ///       - If false, mouse events pass through this window.

src/nvim/ex_docmd.c

@@ -5507,6 +5507,8 @@ static void ex_tabs(exarg_T *eap)
     FOR_ALL_WINDOWS_IN_TAB(wp, tp) {
       if (got_int) {
         break;
+      } else if (!wp->w_config.focusable) {
+        continue;
       }
 
       msg_putchar('\n');

src/nvim/statusline.c

@@ -760,7 +760,9 @@ void draw_tabline(void)
       bool modified = false;
 
       for (wincount = 0; wp != NULL; wp = wp->w_next, wincount++) {
-        if (bufIsChanged(wp->w_buffer)) {
+        if (!wp->w_config.focusable) {
+          wincount--;
+        } else if (bufIsChanged(wp->w_buffer)) {
           modified = true;
         }
       }