Codechange: Don't update window contents if scrollbar position has not moved.

(cherry picked from commit 52b16237ad)

src/widget.cpp

@@ -108,6 +108,7 @@ static void ScrollbarClickPositioning(Window *w, NWidgetScrollbar *sb, int x, in
 	int pos;
 	int button_size;
 	bool rtl = false;
+	bool changed = false;
 
 	if (sb->type == NWID_HSCROLLBAR) {
 		pos = x;
@@ -122,7 +123,7 @@ static void ScrollbarClickPositioning(Window *w, NWidgetScrollbar *sb, int x, in
 		SetBit(sb->disp_flags, NDB_SCROLLBAR_UP);
 		if (_scroller_click_timeout <= 1) {
 			_scroller_click_timeout = 3;
-			sb->UpdatePosition(rtl ? 1 : -1);
+			changed = sb->UpdatePosition(rtl ? 1 : -1);
 		}
 		w->mouse_capture_widget = sb->index;
 	} else if (pos >= ma - button_size) {
@@ -131,16 +132,16 @@ static void ScrollbarClickPositioning(Window *w, NWidgetScrollbar *sb, int x, in
 
 		if (_scroller_click_timeout <= 1) {
 			_scroller_click_timeout = 3;
-			sb->UpdatePosition(rtl ? -1 : 1);
+			changed = sb->UpdatePosition(rtl ? -1 : 1);
 		}
 		w->mouse_capture_widget = sb->index;
 	} else {
 		Point pt = HandleScrollbarHittest(sb, mi, ma, sb->type == NWID_HSCROLLBAR);
 
 		if (pos < pt.x) {
-			sb->UpdatePosition(rtl ? 1 : -1, Scrollbar::SS_BIG);
+			changed = sb->UpdatePosition(rtl ? 1 : -1, Scrollbar::SS_BIG);
 		} else if (pos > pt.y) {
-			sb->UpdatePosition(rtl ? -1 : 1, Scrollbar::SS_BIG);
+			changed = sb->UpdatePosition(rtl ? -1 : 1, Scrollbar::SS_BIG);
 		} else {
 			_scrollbar_start_pos = pt.x - mi - button_size;
 			_scrollbar_size = ma - mi - button_size * 2;
@@ -149,7 +150,13 @@ static void ScrollbarClickPositioning(Window *w, NWidgetScrollbar *sb, int x, in
 		}
 	}
 
-	w->SetDirty();
+	if (changed) {
+		/* Position changed so refresh the window */
+		w->SetDirty();
+	} else {
+		/* No change so only refresh this scrollbar */
+		sb->SetDirty(w);
+	}
 }
 
 /**

src/widget_type.h

@@ -763,12 +763,15 @@ public:
 	/**
 	 * Sets the position of the first visible element
 	 * @param position the position of the element
+	 * @return true iff the position has changed
 	 */
-	void SetPosition(int position)
+	bool SetPosition(int position)
 	{
 		assert(position >= 0);
 		assert(this->count <= this->cap ? (position == 0) : (position + this->cap <= this->count));
+		uint16 old_pos = this->pos;
 		this->pos = position;
+		return this->pos != old_pos;
 	}
 
 	/**
@@ -776,16 +779,17 @@ public:
 	 * If the position would be too low or high it will be clamped appropriately
 	 * @param difference the amount of change requested
 	 * @param unit The stepping unit of \a difference
+	 * @return true iff the position has changed
 	 */
-	void UpdatePosition(int difference, ScrollbarStepping unit = SS_SMALL)
+	bool UpdatePosition(int difference, ScrollbarStepping unit = SS_SMALL)
 	{
-		if (difference == 0) return;
+		if (difference == 0) return false;
 		switch (unit) {
 			case SS_SMALL: difference *= this->stepsize; break;
 			case SS_BIG:   difference *= this->cap; break;
 			default: break;
 		}
-		this->SetPosition(Clamp(this->pos + difference, 0, std::max(this->count - this->cap, 0)));
+		return this->SetPosition(Clamp(this->pos + difference, 0, std::max(this->count - this->cap, 0)));
 	}
 
 	/**

src/window.cpp

@@ -862,8 +862,7 @@ static void DispatchMouseWheelEvent(Window *w, NWidgetCore *nwid, int wheel)
 	if (nwid->type == NWID_VSCROLLBAR) {
 		NWidgetScrollbar *sb = static_cast<NWidgetScrollbar *>(nwid);
 		if (sb->GetCount() > sb->GetCapacity()) {
-			sb->UpdatePosition(wheel);
-			w->SetDirty();
+			if (sb->UpdatePosition(wheel)) w->SetDirty();
 		}
 		return;
 	}
@@ -871,8 +870,7 @@ static void DispatchMouseWheelEvent(Window *w, NWidgetCore *nwid, int wheel)
 	/* Scroll the widget attached to the scrollbar. */
 	Scrollbar *sb = (nwid->scrollbar_index >= 0 ? w->GetScrollbar(nwid->scrollbar_index) : nullptr);
 	if (sb != nullptr && sb->GetCount() > sb->GetCapacity()) {
-		sb->UpdatePosition(wheel);
-		w->SetDirty();
+		if (sb->UpdatePosition(wheel)) w->SetDirty();
 	}
 }
 
@@ -2445,8 +2443,7 @@ static void HandleScrollbarScrolling(Window *w)
 	if (sb->disp_flags & ND_SCROLLBAR_BTN) {
 		if (_scroller_click_timeout == 1) {
 			_scroller_click_timeout = 3;
-			sb->UpdatePosition(rtl == HasBit(sb->disp_flags, NDB_SCROLLBAR_UP) ? 1 : -1);
-			w->SetDirty();
+			if (sb->UpdatePosition(rtl == HasBit(sb->disp_flags, NDB_SCROLLBAR_UP) ? 1 : -1)) w->SetDirty();
 		}
 		return;
 	}