UI: ignore events in empty region overlap areas

- Resizable areas use 2D view bounds.
- Header uses the button bounds.
- A margin is added to avoid clicking between buttons.
- Region resize edges clamp to the 2D view bounds.

Resovles T61554

source/blender/editors/include/UI_interface.h

@@ -86,6 +86,14 @@ typedef struct uiPopupBlockHandle uiPopupBlockHandle;
 #define UI_MAX_NAME_STR 128
 #define UI_MAX_SHORTCUT_STR 64
 
+/**
+ * For #ARegion.overlap regions, pass events though if they don't overlap
+ * the regions contents (the usable part of the #View2D and buttons).
+ *
+ * The margin is needed so it's not possible to accidentally click inbetween buttons.
+ */
+#define UI_REGION_OVERLAP_MARGIN (U.widget_unit / 3)
+
 /* use for clamping popups within the screen */
 #define UI_SCREEN_MARGIN 10
 
@@ -2302,6 +2310,7 @@ void UI_context_active_but_prop_get_templateID(struct bContext *C,
 struct ID *UI_context_active_but_get_tab_ID(struct bContext *C);
 
 uiBut *UI_region_active_but_get(struct ARegion *ar);
+uiBut *UI_region_but_find_rect_over(const struct ARegion *ar, const struct rcti *isect);
 
 /* uiFontStyle.align */
 typedef enum eFontStyle_Align {