floats: implement floating windows

Co-Author: Dongdong Zhou <dzhou121@gmail.com>

src/nvim/api/window.c

@@ -432,3 +432,41 @@ Boolean nvim_win_is_valid(Window window)
   return ret;
 }
 
+
+/// Configure window position. Currently this is only used to configure
+/// floating and external windows (including changing a split window to these
+/// types).
+///
+/// See documentation at |nvim_open_win()|, for the meaning of parameters. Pass
+/// in -1 for 'witdh' and 'height' to keep exiting size.
+///
+/// When reconfiguring a floating window, absent option keys will not be
+/// changed. The following restriction apply: `row`, `col` and `relative`
+/// must be reconfigured together. Only changing a subset of these is an error.
+void nvim_win_config(Window window, Integer width, Integer height,
+                     Dictionary options, Error *err)
+  FUNC_API_SINCE(6)
+{
+  win_T *win = find_window_by_handle(window, err);
+  if (!win) {
+    return;
+  }
+  bool new_float = !win->w_floating;
+  width = width > 0 ? width: win->w_width;
+  height = height > 0 ? height : win->w_height;
+  // reuse old values, if not overriden
+  FloatConfig config = new_float ? FLOAT_CONFIG_INIT : win->w_float_config;
+
+  if (!parse_float_config(options, &config, !new_float, err)) {
+    return;
+  }
+  if (new_float) {
+    if (!win_new_float(win, (int)width, (int)height, config, err)) {
+      return;
+    }
+    redraw_later(NOT_VALID);
+  } else {
+    win_config_float(win, (int)width, (int)height, config);
+    win->w_pos_changed = true;
+  }
+}