Docs: add docstring for wmWindowManger.winactive

Also justify rounding up font width.

source/blender/editors/interface/interface_style.c

@@ -400,6 +400,8 @@ int UI_fontstyle_string_width_with_block_aspect(const uiFontStyle *fs,
   int width = UI_fontstyle_string_width(fs, str);
 
   if (aspect != 1.0f) {
+    /* While in most cases rounding up isn't important, it can make a difference
+     * with small fonts (3px or less), zooming out in the node-editor for e.g. */
     width = (int)ceilf(width * aspect);
   }
   return width;

source/blender/makesdna/DNA_windowmanager_types.h

@@ -138,7 +138,14 @@ typedef struct wmWindowManager {
   ID id;
 
   /** Separate active from drawable. */
-  struct wmWindow *windrawable, *winactive;
+  struct wmWindow *windrawable;
+  /**
+   * \note `CTX_wm_window(C)` is usually preferred.
+   * Avoid relying on this where possible as this may become NULL during when handling
+   * events that close or replace windows (opening a file for e.g.).
+   * While this happens rarely in practice, it can cause difficult to reproduce bugs.
+   */
+  struct wmWindow *winactive;
   ListBase windows;
 
   /** Set on file read. */