[tweak/stable-cursor] Keep stable cursor when viewport is near enough

Allow the viewport to move a bit and still keep the cursor location.

docs/Plugins.rst

@@ -442,7 +442,8 @@ Subcommands that persist until disabled or DF quits:
 :nestbox-color:         Fixes the color of built nestboxes
 :reaction-gloves:       Fixes reactions to produce gloves in sets with correct handedness (:bug:`6273`)
 :shift-8-scroll:        Gives Shift-8 (or :kbd:`*`) priority when scrolling menus, instead of scrolling the map
-:stable-cursor:         Saves the exact cursor position between t/q/k/d/b/etc menus of fortress mode.
+:stable-cursor:         Saves the exact cursor position between t/q/k/d/b/etc menus of fortress mode, if the
+                        map view is near enough to its previous position.
 :stone-status-all:      Adds an option to toggle the economic status of all stones
 :title-start-rename:    Adds a safe rename option to the title screen "Start Playing" menu
 :tradereq-pet-gender:   Displays pet genders on the trade request screen

docs/changelog.txt

@@ -56,6 +56,7 @@ changelog.txt uses a syntax similar to RST, with a few special sequences:
 - `autochop`: only designate the amount of trees required to reach ``max_logs``
 - `autochop`: preferably designate larger trees over smaller ones
 - `blueprint`: ``track`` phase renamed to ``carve``. carved fortifications and (optionally) engravings are now captured in blueprints
+- `tweak` stable-cursor: Keep the cursor stable even when the viewport moves a small amount
 
 ## Documentation
 - Add more examples to the plugin skeleton files so they are more informative for a newbie

plugins/tweak/tweaks/stable-cursor.h

@@ -32,6 +32,26 @@ struct stable_cursor_hook : df::viewscreen_dwarfmodest
         }
     }
 
+    bool check_viewport_near_enough()
+    {
+        df::coord view = Gui::getViewportPos();
+        auto dims = Gui::getDwarfmodeViewDims();
+        int view_size = min(dims.map_x2 - dims.map_x1, dims.map_y2 - dims.map_y1);
+        // Heuristic to keep the cursor position if the view is still in the
+        // part of the map that was already visible
+        int near_enough_distance = min(30, view_size / 2);
+
+        return
+            // Is the viewport near enough to the old viewport?
+            abs(view.x - last_view.x) <= near_enough_distance &&
+            abs(view.y - last_view.y) <= near_enough_distance &&
+            view.z == last_view.z &&
+            // And is the last cursor visible in the current viewport?
+            last_cursor.x >= view.x && last_cursor.y >= view.y &&
+            last_cursor.x <= view.x + dims.map_x2 - dims.map_x1 &&
+            last_cursor.y <= view.y + dims.map_y2 - dims.map_y1;
+    }
+
     DEFINE_VMETHOD_INTERPOSE(void, feed, (set<df::interface_key> *input))
     {
         bool was_default = check_default();
@@ -47,9 +67,8 @@ struct stable_cursor_hook : df::viewscreen_dwarfmodest
         {
             last_view = view; last_cursor = cursor;
         }
-        else if (!is_default && was_default &&
+        else if (!is_default && was_default && cur_cursor.isValid() &&
-                 Gui::getViewportPos() == last_view &&
+                 last_cursor.isValid() && check_viewport_near_enough())
-                 last_cursor.isValid() && cur_cursor.isValid())
         {
             Gui::setCursorCoords(last_cursor.x, last_cursor.y, last_cursor.z);
             Gui::refreshSidebar();