Display: Don't move the center of the viewport when zooming in when the map is narrower/shorter than the viewport

In the test scenario, toggling between 50%/100% zoom on 1920x1080 will
now correctly switch between "Show the entire map" (50%) and "Show just
the center of the map" (100%).

Fixes #1951

src/display.cpp

@@ -2016,13 +2016,28 @@ bool display::set_zoom(unsigned int amount, const bool validate_value_and_set_in
 		zoom_index_ = std::distance(zoom_levels.begin(), iter);
 	}
 
+	const SDL_Rect& outside_area = map_outside_area();
 	const SDL_Rect& area = map_area();
 
 	// Turn the zoom factor to a double in order to avoid rounding errors.
 	double zoom_factor = static_cast<double>(new_zoom) / static_cast<double>(zoom_);
 
+	// INVARIANT: xpos_ + area.w == xend where xend is as in bounds_check_position()
+	//
+	// xpos_: Position of the leftmost visible map pixel of the viewport, in pixels.
+	// Affected by the current zoom: this->zoom_ pixels to the hex.
+	//
+	// xpos_ + area.w/2: Position of the center of the viewport, in pixels.
+	//
+	// (xpos_ + area.w/2) * new_zoom/zoom_: Position of the center of the
+	// viewport, as it would be under new_zoom.
+	//
+	// (xpos_ + area.w/2) * new_zoom/zoom_ - area.w/2: Position of the
+	// leftmost visible map pixel, as it would be under new_zoom.
 	xpos_ = std::round(((xpos_ + area.w / 2) * zoom_factor) - (area.w / 2));
 	ypos_ = std::round(((ypos_ + area.h / 2) * zoom_factor) - (area.h / 2));
+	xpos_ -= (outside_area.w - area.w) / 2;
+	ypos_ -= (outside_area.h - area.h) / 2;
 
 	zoom_ = new_zoom;
 	bounds_check_position(xpos_, ypos_);

src/display.hpp

@@ -733,11 +733,18 @@ protected:
 	CVideo& screen_;
 	std::size_t currentTeam_;
 	bool dont_show_all_; //const team *viewpoint_;
+	/// Position of the top-left corner of the viewport, in pixels.
+	///
+	/// Dependent on zoom_.. For example, ypos_==72 only means we're one
+	/// hex below the top of the map when zoom_ == 72 (the default value).
 	int xpos_, ypos_;
 	bool view_locked_;
 	theme theme_;
+	/// The current zoom, in pixels (on screen) per 72 pixels (in the
+	/// graphic assets), i.e., 72 means 100%.
 	static unsigned int zoom_;
 	int zoom_index_;
+	/// The previous value of zoom_.
 	static unsigned int last_zoom_;
 	const std::unique_ptr<fake_unit_manager> fake_unit_man_;
 	const std::unique_ptr<terrain_builder> builder_;

src/map/map.hpp

@@ -121,10 +121,10 @@ public:
 			bool is_odd,
 			bool ignore_special_locations);
 
-	/** Effective map width. */
+	/** Effective map width, in hexes. */
 	int w() const { return w_; }
 
-	/** Effective map height. */
+	/** Effective map height, in hexes. */
 	int h() const { return h_; }
 
 	/** Size of the map border. */