UI: Don't spawn asset shelf popup by hovering button

Papercut reported in #132293.

Allow explicitly disabling this behavior for a button, and do so for the
big preview asset shelf popup button. It gets more in the way than it's
useful in this case.

source/blender/editors/include/UI_interface_c.hh

@@ -1830,6 +1830,13 @@ void UI_but_func_drawextra_set(uiBlock *block,
 
 void UI_but_func_menu_step_set(uiBut *but, uiMenuStepFunc func);
 
+/**
+ * When a button displays a menu, hovering another button that can display one will switch to that
+ * menu instead. In some cases that's unexpected, so the feature can be disabled here (as in, this
+ * button will not spawn its menu on hover and the previously spawned menu will remain open).
+ */
+void UI_but_menu_disable_hover_open(uiBut *but);
+
 void UI_but_func_tooltip_set(uiBut *but, uiButToolTipFunc func, void *arg, uiFreeArgFunc free_arg);
 /**
  * Enable a custom quick tooltip label. That is, a short tooltip that appears faster than the full

source/blender/editors/interface/interface.cc

@@ -6196,6 +6196,11 @@ void UI_but_func_menu_step_set(uiBut *but, uiMenuStepFunc func)
   but->menu_step_func = func;
 }
 
+void UI_but_menu_disable_hover_open(uiBut *but)
+{
+  but->menu_no_hover_open = true;
+}
+
 void UI_but_func_tooltip_label_set(uiBut *but, std::function<std::string(const uiBut *but)> func)
 {
   but->tip_label_func = std::move(func);

source/blender/editors/interface/interface_handlers.cc

@@ -11965,6 +11965,7 @@ static int ui_handler_region_menu(bContext *C, const wmEvent *event, void * /*us
         ELEM(but->type, UI_BTYPE_PULLDOWN, UI_BTYPE_POPOVER, UI_BTYPE_MENU) &&
         (but_other = ui_but_find_mouse_over(region, event)) && (but != but_other) &&
         ELEM(but_other->type, UI_BTYPE_PULLDOWN, UI_BTYPE_POPOVER, UI_BTYPE_MENU) &&
+        !but_other->menu_no_hover_open &&
         /* Hover-opening menu's doesn't work well for buttons over one another
          * along the same axis the menu is opening on (see #71719). */
         (((data->menu->direction & (UI_DIR_LEFT | UI_DIR_RIGHT)) &&

source/blender/editors/interface/interface_intern.hh

@@ -269,6 +269,8 @@ struct uiBut {
   uiMenuCreateFunc menu_create_func = nullptr;
 
   uiMenuStepFunc menu_step_func = nullptr;
+  /** See #UI_but_menu_disable_hover_open(). */
+  bool menu_no_hover_open = false;
 
   /* RNA data */
   PointerRNA rnapoin = {};

source/blender/editors/interface/templates/interface_template_asset_shelf_popover.cc

@@ -57,6 +57,9 @@ void template_asset_shelf_popover(uiLayout &layout,
   uiBut *but = static_cast<uiBut *>(block->buttons.last);
   if (use_preview_icon) {
     ui_def_but_icon(but, icon, UI_HAS_ICON | UI_BUT_ICON_PREVIEW);
+    /* Avoid small annoyance where asset shelf popover gets spawned unintentionally on mouse hover,
+     * see #132293. */
+    UI_but_menu_disable_hover_open(but);
   }
 }