Implement stickyCursor

This setting emulates vim's limitation (or feature, depending on how you look at it) which kept the cursor within the viewport, even when scrolling with the mouse.
Fixes #873, #3846

extension.ts

@@ -292,6 +292,26 @@ export async function activate(context: vscode.ExtensionContext) {
     true
   );
 
+  registerEventListener(context, vscode.window.onDidChangeTextEditorVisibleRanges, async event => {
+    if (configuration.stickyCursor) {
+      taskQueue.enqueueTask(async () => {
+        const viewPort = event.visibleRanges[0];
+        const mh = await getAndUpdateModeHandler();
+        if (viewPort.start.line > mh.vimState.cursorStopPosition.line) {
+          await vscode.commands.executeCommand('cursorMove', {
+            to: 'viewPortTop',
+          });
+          mh.syncCursors();
+        } else if (mh.vimState.cursorStopPosition.line > viewPort.end.line) {
+          await vscode.commands.executeCommand('cursorMove', {
+            to: 'viewPortBottom',
+          });
+          mh.syncCursors();
+        }
+      });
+    }
+  });
+
   const compositionState = new CompositionState();
 
   // override vscode commands

package.json

@@ -831,6 +831,11 @@
           "type": "boolean",
           "description": "Searches wrap around the end of the file.",
           "default": true
+        },
+        "vim.stickyCursor": {
+          "type": "boolean",
+          "description": "If true, the cursor will always be kept within the viewport.",
+          "default": false
         }
       }
     }

src/configuration/configuration.ts

@@ -185,6 +185,8 @@ class Configuration implements IConfiguration {
   sneakUseIgnorecaseAndSmartcase = false;
   sneakReplacesF = false;
 
+  stickyCursor = false;
+
   surround = true;
 
   easymotion = false;