Fix: volume slider behavior in music gui

In the music gui, the volume sliders don't respond to mouse drags
that extend outside of their small area.

This changes the event code to give them more typical dragging behavior
that's easier to work with.

src/music_gui.cpp

@@ -26,6 +26,7 @@
 #include "string_func.h"
 #include "settings_type.h"
 #include "settings_gui.h"
+#include "tilehighlight_func.h"
 #include "widgets/dropdown_func.h"
 #include "widgets/dropdown_type.h"
 
@@ -650,6 +651,7 @@ static void ShowMusicTrackSelection()
 
 struct MusicWindow : public Window {
 	static const int slider_width = 3;
+	int dragged_vol_widget = 0;
 
 	MusicWindow(WindowDesc *desc, WindowNumber number) : Window(desc)
 	{
@@ -795,22 +797,14 @@ struct MusicWindow : public Window {
 				break;
 
 			case WID_M_MUSIC_VOL: case WID_M_EFFECT_VOL: { // volume sliders
-				int x = pt.x - this->GetWidget<NWidgetBase>(widget)->pos_x;
-
-				byte *vol = (widget == WID_M_MUSIC_VOL) ? &_settings_client.music.music_vol : &_settings_client.music.effect_vol;
-
-				byte new_vol = x * 127 / this->GetWidget<NWidgetBase>(widget)->current_x;
-				if (_current_text_dir == TD_RTL) new_vol = 127 - new_vol;
-				/* Clamp to make sure min and max are properly settable */
-				if (new_vol > 124) new_vol = 127;
-				if (new_vol < 3) new_vol = 0;
-				if (new_vol != *vol) {
-					*vol = new_vol;
-					if (widget == WID_M_MUSIC_VOL) MusicDriver::GetInstance()->SetVolume(new_vol);
-					this->SetDirty();
-				}
+				this->dragged_vol_widget = widget;
+
+				this->UpdateVolumeWidget(pt);
 
 				_left_button_clicked = false;
+
+				/* Activate dragging */
+				SetObjectToPlaceWnd(SPR_CURSOR_MOUSE, PAL_NONE, HT_DRAG, this);
 				break;
 			}
 
@@ -834,6 +828,35 @@ struct MusicWindow : public Window {
 				break;
 		}
 	}
+
+	virtual void OnMouseDrag(Point pt, int widget)
+	{
+		this->UpdateVolumeWidget(pt);
+	}
+
+	void UpdateVolumeWidget(Point pt)
+	{
+		uint widget_width = this->GetWidget<NWidgetBase>(this->dragged_vol_widget)->current_x;
+
+		int x = pt.x - this->GetWidget<NWidgetBase>(this->dragged_vol_widget)->pos_x;
+
+		/* Only update the volume along the width of this widget */
+		if (x < 0) x = 0;
+		if (x > (int)widget_width) x = (int)widget_width;
+
+		byte *vol = (this->dragged_vol_widget == WID_M_MUSIC_VOL) ? &_settings_client.music.music_vol : &_settings_client.music.effect_vol;
+
+		byte new_vol = x * 127 / widget_width;
+		if (_current_text_dir == TD_RTL) new_vol = 127 - new_vol;
+		/* Clamp to make sure min and max are properly settable */
+		if (new_vol > 124) new_vol = 127;
+		if (new_vol < 3) new_vol = 0;
+		if (new_vol != *vol) {
+			*vol = new_vol;
+			if (this->dragged_vol_widget == WID_M_MUSIC_VOL) MusicDriver::GetInstance()->SetVolume(new_vol);
+			this->SetDirty();
+		}
+	}
 };
 
 static const NWidgetPart _nested_music_window_widgets[] = {