Fix T76635: Clicking the text button X in a pop-up doesn't clear

In this case giving `CTX_wm_menu()` priority over `CTX_wm_region()` is all
that's needed and makes sense (since we want exactly the hovered button, not
some other active button in the region/menu hierarchy).

The situation with pop-ups is still tricky, see T73565. But as a first step
it's probably good to let functions be more explicit about what they want when
querying UI context. So I added a variation of a UI-context function for cases
like this.

source/blender/editors/include/UI_interface.h

@@ -2411,6 +2411,7 @@ bool UI_context_copy_to_selected_list(struct bContext *C,
 
 /* Helpers for Operators */
 uiBut *UI_context_active_but_get(const struct bContext *C);
+uiBut *UI_context_active_but_get_respect_menu(const struct bContext *C);
 uiBut *UI_context_active_but_prop_get(const struct bContext *C,
                                       struct PointerRNA *r_ptr,
                                       struct PropertyRNA **r_prop,

source/blender/editors/interface/interface_handlers.c

@@ -8259,11 +8259,22 @@ static uiBut *ui_context_rna_button_active(const bContext *C)
   return ui_context_button_active(CTX_wm_region(C), ui_context_rna_button_active_test);
 }
 
-uiBut *UI_context_active_but_get(const struct bContext *C)
+uiBut *UI_context_active_but_get(const bContext *C)
 {
   return ui_context_button_active(CTX_wm_region(C), NULL);
 }
 
+/*
+ * Version of #UI_context_active_get() that uses the result of #CTX_wm_menu()
+ * if set. Does not traverse into parent menus, which may be wanted in some
+ * cases.
+ */
+uiBut *UI_context_active_but_get_respect_menu(const bContext *C)
+{
+  ARegion *ar_menu = CTX_wm_menu(C);
+  return ui_context_button_active(ar_menu ? ar_menu : CTX_wm_region(C), NULL);
+}
+
 uiBut *UI_region_active_but_get(ARegion *region)
 {
   return ui_context_button_active(region, NULL);

source/blender/editors/interface/interface_ops.c

@@ -1679,7 +1679,7 @@ static void UI_OT_button_execute(wmOperatorType *ot)
 
 static int button_string_clear_exec(bContext *C, wmOperator *UNUSED(op))
 {
-  uiBut *but = UI_context_active_but_get(C);
+  uiBut *but = UI_context_active_but_get_respect_menu(C);
 
   if (but) {
     ui_but_active_string_clear_and_exit(C, but);