Add SCROLL_MODE_RESERVE to ScrollContainer

doc/classes/ScrollContainer.xml

@@ -102,6 +102,9 @@
 		<constant name="SCROLL_MODE_SHOW_NEVER" value="3" enum="ScrollMode">
 			Scrolling enabled, scrollbar will be hidden.
 		</constant>
+		<constant name="SCROLL_MODE_RESERVE" value="4" enum="ScrollMode">
+			Combines [constant SCROLL_MODE_AUTO] and [constant SCROLL_MODE_SHOW_ALWAYS]. The scrollbar is only visible if necessary, but the content size is adjusted as if it was always visible. It's useful for ensuring that content size stays the same regardless if the scrollbar is visible.
+		</constant>
 	</constants>
 	<theme_items>
 		<theme_item name="panel" data_type="style" type="StyleBox">

scene/gui/scroll_container.cpp

@@ -58,7 +58,7 @@ Size2 ScrollContainer::get_minimum_size() const {
 
 	if (horizontal_scroll_mode == SCROLL_MODE_DISABLED) {
 		min_size.x = largest_child_min_size.x;
-		bool v_scroll_show = vertical_scroll_mode == SCROLL_MODE_SHOW_ALWAYS || (vertical_scroll_mode == SCROLL_MODE_AUTO && largest_child_min_size.y > size.y);
+		bool v_scroll_show = vertical_scroll_mode == SCROLL_MODE_SHOW_ALWAYS || vertical_scroll_mode == SCROLL_MODE_RESERVE || (vertical_scroll_mode == SCROLL_MODE_AUTO && largest_child_min_size.y > size.y);
 		if (v_scroll_show && v_scroll->get_parent() == this) {
 			min_size.x += v_scroll->get_minimum_size().x;
 		}
@@ -66,7 +66,7 @@ Size2 ScrollContainer::get_minimum_size() const {
 
 	if (vertical_scroll_mode == SCROLL_MODE_DISABLED) {
 		min_size.y = largest_child_min_size.y;
-		bool h_scroll_show = horizontal_scroll_mode == SCROLL_MODE_SHOW_ALWAYS || (horizontal_scroll_mode == SCROLL_MODE_AUTO && largest_child_min_size.x > size.x);
+		bool h_scroll_show = horizontal_scroll_mode == SCROLL_MODE_SHOW_ALWAYS || horizontal_scroll_mode == SCROLL_MODE_RESERVE || (horizontal_scroll_mode == SCROLL_MODE_AUTO && largest_child_min_size.x > size.x);
 		if (h_scroll_show && h_scroll->get_parent() == this) {
 			min_size.y += h_scroll->get_minimum_size().y;
 		}
@@ -92,6 +92,15 @@ void ScrollContainer::_cancel_drag() {
 	}
 }
 
+bool ScrollContainer::_is_h_scroll_visible() const {
+	// Scrolls may have been moved out for reasons.
+	return h_scroll->is_visible() && h_scroll->get_parent() == this;
+}
+
+bool ScrollContainer::_is_v_scroll_visible() const {
+	return v_scroll->is_visible() && v_scroll->get_parent() == this;
+}
+
 void ScrollContainer::gui_input(const Ref<InputEvent> &p_gui_input) {
 	ERR_FAIL_COND(p_gui_input.is_null());
 
@@ -298,11 +307,11 @@ void ScrollContainer::_reposition_children() {
 	ofs += theme_cache.panel_style->get_offset();
 	bool rtl = is_layout_rtl();
 
-	if (h_scroll->is_visible_in_tree() && h_scroll->get_parent() == this) { //scrolls may have been moved out for reasons
+	if (_is_h_scroll_visible() || horizontal_scroll_mode == SCROLL_MODE_RESERVE) {
 		size.y -= h_scroll->get_minimum_size().y;
 	}
 
-	if (v_scroll->is_visible_in_tree() && v_scroll->get_parent() == this) { //scrolls may have been moved out for reasons
+	if (_is_v_scroll_visible() || vertical_scroll_mode == SCROLL_MODE_RESERVE) {
 		size.x -= v_scroll->get_minimum_size().x;
 	}
 
@@ -324,7 +333,7 @@ void ScrollContainer::_reposition_children() {
 			r.size.height = MAX(size.height, minsize.height);
 		}
 		r.position += ofs;
-		if (rtl && v_scroll->is_visible_in_tree() && v_scroll->get_parent() == this) {
+		if (rtl && _is_v_scroll_visible()) {
 			r.position.x += v_scroll->get_minimum_size().x;
 		}
 		r.position = r.position.floor();
@@ -436,14 +445,14 @@ void ScrollContainer::update_scrollbars() {
 	Size2 hmin = h_scroll->get_combined_minimum_size();
 	Size2 vmin = v_scroll->get_combined_minimum_size();
 
-	h_scroll->set_visible(horizontal_scroll_mode == SCROLL_MODE_SHOW_ALWAYS || (horizontal_scroll_mode == SCROLL_MODE_AUTO && largest_child_min_size.width > size.width));
-	v_scroll->set_visible(vertical_scroll_mode == SCROLL_MODE_SHOW_ALWAYS || (vertical_scroll_mode == SCROLL_MODE_AUTO && largest_child_min_size.height > size.height));
+	h_scroll->set_visible(horizontal_scroll_mode == SCROLL_MODE_SHOW_ALWAYS || ((horizontal_scroll_mode == SCROLL_MODE_AUTO || horizontal_scroll_mode == SCROLL_MODE_RESERVE) && largest_child_min_size.width > size.width));
+	v_scroll->set_visible(vertical_scroll_mode == SCROLL_MODE_SHOW_ALWAYS || ((vertical_scroll_mode == SCROLL_MODE_AUTO || vertical_scroll_mode == SCROLL_MODE_RESERVE) && largest_child_min_size.height > size.height));
 
 	h_scroll->set_max(largest_child_min_size.width);
-	h_scroll->set_page((v_scroll->is_visible() && v_scroll->get_parent() == this) ? size.width - vmin.width : size.width);
+	h_scroll->set_page(_is_v_scroll_visible() ? size.width - vmin.width : size.width);
 
 	v_scroll->set_max(largest_child_min_size.height);
-	v_scroll->set_page((h_scroll->is_visible() && h_scroll->get_parent() == this) ? size.height - hmin.height : size.height);
+	v_scroll->set_page(_is_h_scroll_visible() ? size.height - hmin.height : size.height);
 
 	// Avoid scrollbar overlapping.
 	_updating_scrollbars = true;
@@ -603,14 +612,15 @@ void ScrollContainer::_bind_methods() {
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "scroll_vertical", PROPERTY_HINT_NONE, "suffix:px"), "set_v_scroll", "get_v_scroll");
 	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "scroll_horizontal_custom_step", PROPERTY_HINT_RANGE, "-1,4096,suffix:px"), "set_horizontal_custom_step", "get_horizontal_custom_step");
 	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "scroll_vertical_custom_step", PROPERTY_HINT_RANGE, "-1,4096,suffix:px"), "set_vertical_custom_step", "get_vertical_custom_step");
-	ADD_PROPERTY(PropertyInfo(Variant::INT, "horizontal_scroll_mode", PROPERTY_HINT_ENUM, "Disabled,Auto,Always Show,Never Show"), "set_horizontal_scroll_mode", "get_horizontal_scroll_mode");
-	ADD_PROPERTY(PropertyInfo(Variant::INT, "vertical_scroll_mode", PROPERTY_HINT_ENUM, "Disabled,Auto,Always Show,Never Show"), "set_vertical_scroll_mode", "get_vertical_scroll_mode");
+	ADD_PROPERTY(PropertyInfo(Variant::INT, "horizontal_scroll_mode", PROPERTY_HINT_ENUM, "Disabled,Auto,Always Show,Never Show,Reserve"), "set_horizontal_scroll_mode", "get_horizontal_scroll_mode");
+	ADD_PROPERTY(PropertyInfo(Variant::INT, "vertical_scroll_mode", PROPERTY_HINT_ENUM, "Disabled,Auto,Always Show,Never Show,Reserve"), "set_vertical_scroll_mode", "get_vertical_scroll_mode");
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "scroll_deadzone"), "set_deadzone", "get_deadzone");
 
 	BIND_ENUM_CONSTANT(SCROLL_MODE_DISABLED);
 	BIND_ENUM_CONSTANT(SCROLL_MODE_AUTO);
 	BIND_ENUM_CONSTANT(SCROLL_MODE_SHOW_ALWAYS);
 	BIND_ENUM_CONSTANT(SCROLL_MODE_SHOW_NEVER);
+	BIND_ENUM_CONSTANT(SCROLL_MODE_RESERVE);
 
 	BIND_THEME_ITEM_CUSTOM(Theme::DATA_TYPE_STYLEBOX, ScrollContainer, panel_style, "panel");
 

scene/gui/scroll_container.h

@@ -44,6 +44,7 @@ public:
 		SCROLL_MODE_AUTO,
 		SCROLL_MODE_SHOW_ALWAYS,
 		SCROLL_MODE_SHOW_NEVER,
+		SCROLL_MODE_RESERVE,
 	};
 
 private:
@@ -75,6 +76,9 @@ private:
 
 	void _cancel_drag();
 
+	bool _is_h_scroll_visible() const;
+	bool _is_v_scroll_visible() const;
+
 protected:
 	Size2 get_minimum_size() const override;