[Feature] Allow underzooming with single-copy world

src/geo/transform.ts

@@ -785,7 +785,26 @@ export class Transform {
         let maxX = worldSize;
         let scaleY = 0;
         let scaleX = 0;
-        const {x: screenWidth, y: screenHeight} = this.size;
+        let {x: screenWidth, y: screenHeight} = this.size;
+
+        // For a single-world map, a user can "underzoom" the world to see it entirely in the viewport.
+        // This works by reducing the viewport's appararent size to a square with a side length equal to the smallest viewport dimension, then reduced by a factor `boundsRatio`.
+        // `minZoom` is obeyed in all cases.
+        if (!this._renderWorldCopies) {
+            // `boundsRatio` is the percentage of the bounds a user can exceed when panning and/or zooming, where a bound is measured from viewport edge to viewport center.
+            // The default is boundsRatio=0.5, which on a wide viewport means a user can pan and zoom out until the world's top and bottom edges are halfway between the viewport center and its top and bottom edges.
+            // _______________________________
+            // |viewport      : } boundsRatio|
+            // |        -———————————-        |
+            // |        |world:     |        |
+            // |        |     :     |        |
+            // |        |     *     |        |
+            // |        |           |        |
+            // |        -———————————-        |
+            // |_____________________________|
+            const boundsRatio = 0.5;
+            screenWidth = screenHeight = (1.0 - boundsRatio) * Math.min(screenWidth, screenHeight);
+        }
 
         if (this.latRange) {
             const latRange = this.latRange;