Fix left mouse button scroll in viewport map mode

src/viewport.cpp

@@ -4215,15 +4215,15 @@ bool HandleViewportDoubleClicked(Window *w, int x, int y)
 	}
 }
 
-bool HandleViewportClicked(const Viewport *vp, int x, int y, bool double_click)
+HandleViewportClickedResult HandleViewportClicked(const Viewport *vp, int x, int y, bool double_click)
 {
-	/* No click in smallmap mode except for plan making. */
-	if (vp->zoom >= ZOOM_LVL_DRAW_MAP && !(_thd.place_mode & HT_MAP)) return true;
+	/* No click in smallmap mode except for plan making and left-button scrolling. */
+	if (vp->zoom >= ZOOM_LVL_DRAW_MAP && !(_thd.place_mode & HT_MAP)) return HVCR_SCROLL_ONLY;
 
 	const Vehicle *v = CheckClickOnVehicle(vp, x, y);
 
 	if (_thd.place_mode & HT_VEHICLE) {
-		if (v != nullptr && VehicleClicked(v)) return true;
+		if (v != nullptr && VehicleClicked(v)) return HVCR_DENY;
 	}
 
 	/* Vehicle placement mode already handled above. */
@@ -4235,18 +4235,18 @@ bool HandleViewportClicked(const Viewport *vp, int x, int y, bool double_click)
 			static bool stop_snap_on_double_click = false;
 			if (double_click && stop_snap_on_double_click) {
 				SetRailSnapMode(RSM_NO_SNAP);
-				return true;
+				return HVCR_DENY;
 			}
 			stop_snap_on_double_click = !(_thd.drawstyle & HT_LINE) || (_thd.dir2 == HT_DIR_END);
 		}
 
 		PlaceObject();
-		return true;
+		return HVCR_DENY;
 	}
 
-	if (vp->zoom >= ZOOM_LVL_DRAW_MAP) return true;
+	if (vp->zoom >= ZOOM_LVL_DRAW_MAP) return HVCR_SCROLL_ONLY;
 
-	if (CheckClickOnViewportSign(vp, x, y)) return true;
+	if (CheckClickOnViewportSign(vp, x, y)) return HVCR_DENY;
 	bool result = CheckClickOnLandscape(vp, x, y);
 
 	if (v != nullptr) {
@@ -4260,9 +4260,9 @@ bool HandleViewportClicked(const Viewport *vp, int x, int y, bool double_click)
 				ShowVehicleViewWindow(v);
 			}
 		}
-		return true;
+		return HVCR_DENY;
 	}
-	return result;
+	return result ? HVCR_DENY : HVCR_ALLOW;
 }
 
 void RebuildViewportOverlay(Window *w, bool incremental)

src/viewport_func.h

@@ -75,8 +75,14 @@ void ViewportAddString(const DrawPixelInfo *dpi, ZoomLevel small_from, const Vie
 void StartSpriteCombine();
 void EndSpriteCombine();
 
+enum HandleViewportClickedResult {
+	HVCR_DENY,
+	HVCR_SCROLL_ONLY,
+	HVCR_ALLOW,
+};
+
 bool HandleViewportDoubleClicked(Window *w, int x, int y);
-bool HandleViewportClicked(const Viewport *vp, int x, int y, bool double_click);
+HandleViewportClickedResult HandleViewportClicked(const Viewport *vp, int x, int y, bool double_click);
 void SetRedErrorSquare(TileIndex tile);
 void SetTileSelectSize(int w, int h);
 void SetTileSelectBigSize(int ox, int oy, int sx, int sy);

src/window.cpp

@@ -2995,15 +2995,18 @@ static void MouseLoop(MouseClick click, int mousewheel)
 			case MC_DOUBLE_LEFT:
 				if (HandleViewportDoubleClicked(w, x, y)) break;
 				/* FALL THROUGH */
-			case MC_LEFT:
-				if (HandleViewportClicked(vp, x, y, click == MC_DOUBLE_LEFT)) return;
+			case MC_LEFT: {
+				HandleViewportClickedResult result = HandleViewportClicked(vp, x, y, click == MC_DOUBLE_LEFT);
+				if (result == HVCR_DENY) return;
 				if (!(w->flags & WF_DISABLE_VP_SCROLL) &&
 						_settings_client.gui.scroll_mode == VSM_MAP_LMB) {
 					_scrolling_viewport = w;
 					_cursor.fix_at = false;
 					return;
 				}
+				if (result != HVCR_ALLOW) return;
 				break;
+			}
 
 			case MC_RIGHT:
 				if (!(w->flags & WF_DISABLE_VP_SCROLL) &&