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/ED_screen.h

@@ -403,6 +403,18 @@ void ED_region_generic_tools_region_message_subscribe(const struct bContext *C,
                                                       struct wmMsgBus *mbus);
 int ED_region_generic_tools_region_snap_size(const struct ARegion *ar, int size, int axis);
 
+/* area_query.c */
+bool ED_region_overlap_isect_x(const ARegion *ar, const int event_x);
+bool ED_region_overlap_isect_y(const ARegion *ar, const int event_y);
+bool ED_region_overlap_isect_xy(const ARegion *ar, const int event_xy[2]);
+bool ED_region_overlap_isect_x_with_margin(const ARegion *ar, const int event_x, const int margin);
+bool ED_region_overlap_isect_y_with_margin(const ARegion *ar, const int event_y, const int margin);
+bool ED_region_overlap_isect_xy_with_margin(const ARegion *ar,
+                                            const int event_xy[2],
+                                            const int margin);
+
+bool ED_region_contains_xy(const struct ARegion *ar, const int event_xy[2]);
+
 /* interface_region_hud.c */
 struct ARegionType *ED_area_type_hud(int space_type);
 void ED_area_type_hud_clear(struct wmWindowManager *wm, ScrArea *sa_keep);

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 {

source/blender/editors/include/UI_view2d.h

@@ -117,6 +117,9 @@ enum eView2D_Gridlines {
 #define IN_2D_VERT_SCROLL(v2d, co) (BLI_rcti_isect_pt_v(&v2d->vert, co))
 #define IN_2D_HORIZ_SCROLL(v2d, co) (BLI_rcti_isect_pt_v(&v2d->hor, co))
 
+#define IN_2D_VERT_SCROLL_RECT(v2d, rct) (BLI_rcti_isect(&v2d->vert, rct, NULL))
+#define IN_2D_HORIZ_SCROLL_RECT(v2d, rct) (BLI_rcti_isect(&v2d->hor, rct, NULL))
+
 /* ------------------------------------------ */
 /* Type definitions:                          */
 
@@ -275,6 +278,13 @@ char UI_view2d_mouse_in_scrollers(const struct ARegion *ar,
                                   const struct View2D *v2d,
                                   int x,
                                   int y);
+char UI_view2d_rect_in_scrollers_ex(const struct ARegion *ar,
+                                    const struct View2D *v2d,
+                                    const struct rcti *rect,
+                                    int *r_scroll);
+char UI_view2d_rect_in_scrollers(const struct ARegion *ar,
+                                 const struct View2D *v2d,
+                                 const struct rcti *rect);
 
 /* cached text drawing in v2d, to allow pixel-aligned draw as post process */
 void UI_view2d_text_cache_add(