scrollcontainer: Add automatic scrollbar calculation (#14623)

New parameter 'content padding'. When specified, the scrollbar
max value is calculated automatically. This aims to reduce manual
calculation functions.

src/gui/guiFormSpecMenu.cpp

@@ -356,7 +356,7 @@ void GUIFormSpecMenu::parseContainerEnd(parserData* data, const std::string &)
 void GUIFormSpecMenu::parseScrollContainer(parserData *data, const std::string &element)
 {
 	std::vector<std::string> parts;
-	if (!precheckElement("scroll_container start", element, 4, 5, parts))
+	if (!precheckElement("scroll_container start", element, 4, 6, parts))
 		return;
 
 	std::vector<std::string> v_pos  = split(parts[0], ',');
@@ -367,6 +367,12 @@ void GUIFormSpecMenu::parseScrollContainer(parserData *data, const std::string &
 	if (parts.size() >= 5 && !parts[4].empty())
 		scroll_factor = stof(parts[4]);
 
+	std::optional<s32> content_padding_px;
+	if (parts.size() >= 6 && !parts[5].empty()) {
+		std::vector<std::string> v_size = { parts[5], parts[5] };
+		content_padding_px = getRealCoordinateGeometry(v_size)[orientation == "vertical" ? 1 : 0];
+	}
+
 	MY_CHECKPOS("scroll_container", 0);
 	MY_CHECKGEOM("scroll_container", 1);
 
@@ -405,6 +411,7 @@ void GUIFormSpecMenu::parseScrollContainer(parserData *data, const std::string &
 
 	GUIScrollContainer *mover = new GUIScrollContainer(Environment,
 			clipper, spec_mover.fid, rect_mover, orientation, scroll_factor);
+	mover->setContentPadding(content_padding_px);
 
 	data->current_parent = mover;
 

src/gui/guiScrollBar.h

@@ -45,6 +45,7 @@ public:
 	s32 getSmallStep() const { return small_step; }
 	s32 getPos() const;
 	s32 getTargetPos() const;
+	bool isHorizontal() const { return is_horizontal; }
 
 	void setMax(const s32 &max);
 	void setMin(const s32 &min);

src/gui/guiScrollContainer.cpp

@@ -67,6 +67,50 @@ void GUIScrollContainer::draw()
 	}
 }
 
+void GUIScrollContainer::setScrollBar(GUIScrollBar *scrollbar)
+{
+	m_scrollbar = scrollbar;
+
+	if (m_scrollbar && m_content_padding_px.has_value() && m_scrollfactor != 0.0f) {
+		// Set the scrollbar max value based on the content size.
+
+		// Get content size based on elements
+		core::rect<s32> size;
+		for (gui::IGUIElement *e : Children) {
+			core::rect<s32> abs_rect = e->getAbsolutePosition();
+			size.addInternalPoint(abs_rect.LowerRightCorner);
+		}
+
+		s32 visible_content_px = (
+			m_orientation == VERTICAL
+				? AbsoluteClippingRect.getHeight()
+				: AbsoluteClippingRect.getWidth()
+		);
+
+		s32 total_content_px = *m_content_padding_px + (
+			m_orientation == VERTICAL
+				? (size.LowerRightCorner.Y - AbsoluteClippingRect.UpperLeftCorner.Y)
+				: (size.LowerRightCorner.X - AbsoluteClippingRect.UpperLeftCorner.X)
+		);
+
+		s32 hidden_content_px = std::max<s32>(0, total_content_px - visible_content_px);
+		m_scrollbar->setMin(0);
+		m_scrollbar->setMax(std::ceil(hidden_content_px / std::fabs(m_scrollfactor)));
+
+		// Note: generally, the scrollbar has the same size as the scroll container.
+		// However, in case it isn't, proportional adjustments are needed.
+		s32 scrollbar_px = (
+			m_scrollbar->isHorizontal()
+				? m_scrollbar->getRelativePosition().getWidth()
+				: m_scrollbar->getRelativePosition().getHeight()
+		);
+
+		m_scrollbar->setPageSize((total_content_px * scrollbar_px) / visible_content_px);
+	}
+
+	updateScrolling();
+}
+
 void GUIScrollContainer::updateScrolling()
 {
 	s32 pos = m_scrollbar->getPos();

src/gui/guiScrollContainer.h

@@ -34,17 +34,18 @@ public:
 
 	virtual void draw() override;
 
+	inline void setContentPadding(std::optional<s32> padding)
+	{
+		m_content_padding_px = padding;
+	}
+
 	inline void onScrollEvent(gui::IGUIElement *caller)
 	{
 		if (caller == m_scrollbar)
 			updateScrolling();
 	}
 
-	inline void setScrollBar(GUIScrollBar *scrollbar)
-	{
-		m_scrollbar = scrollbar;
-		updateScrolling();
-	}
+	void setScrollBar(GUIScrollBar *scrollbar);
 
 private:
 	enum OrientationEnum
@@ -56,7 +57,8 @@ private:
 
 	GUIScrollBar *m_scrollbar;
 	OrientationEnum m_orientation;
-	f32 m_scrollfactor;
+	f32 m_scrollfactor; //< scrollbar pos * scrollfactor = scroll offset in pixels
+	std::optional<s32> m_content_padding_px; //< in pixels
 
 	void updateScrolling();
 };