feat(workingset): switch files on mouse down for faster perceived switching

The file tree and editor tab bar switch files on mouse down, but the
working set only switched on mouse up, making it feel slow. Open the file
immediately on left-button press (unless the press is on the close icon)
and skip the duplicate open in the mouse up drop handler. Right/middle
click paths already resolved at mouse down and are unchanged; close via
the close icon still happens on mouse up without selecting the file.

A drag now makes the pressed item the current file as soon as it is
pressed, consistent with dragging tabs in other editors.

Claude-Session: https://claude.ai/code/session_018BpQ3Z7w4H2U5qrPGWYRWB

Author: abose

src/project/WorkingSetView.js

@@ -309,7 +309,8 @@ define(function (require, exports, module) {
                 offset,
                 $copy,
                 $ghost,
-                draggingCurrentFile;
+                draggingCurrentFile,
+                openedOnMouseDown = false;
 
             function initDragging() {
                 itemHeight = $el.height();
@@ -789,6 +790,10 @@ define(function (require, exports, module) {
                                 .always(function () {
                                     postDropCleanup();
                                 });
+                        } else if (openedOnMouseDown) {
+                            // file was already opened on mouse down for a fast switch feel,
+                            // nothing left to do here
+                            postDropCleanup();
                         } else {
                             // Normal right and left click - select the item
                             FileViewController.setFileViewFocus(FileViewController.WORKING_SET_VIEW);
@@ -864,7 +869,16 @@ define(function (require, exports, module) {
                 return;
             }
 
-
+            // open the file right away on mouse down like the file tree and the
+            // editor tab bar do, so switching files feels fast instead of waiting
+            // for mouse up. A drag can still follow - the item just becomes the
+            // current file as soon as it is pressed.
+            if (!tryClosing) {
+                openedOnMouseDown = true;
+                FileViewController.setFileViewFocus(FileViewController.WORKING_SET_VIEW);
+                CommandManager.execute(Commands.FILE_OPEN, {fullPath: sourceFile.fullPath,
+                    paneId: sourceView.paneId});
+            }
 
             e.stopPropagation();
         });