feat: don't show the UI on very fast shortcut triggers from the user

By listening to keyboard events from another thread, that thread is able to cancel tasks scheduled on the main UI thread. This means that if the user activates the app, and very quickly release the keys to focus the other window, that key release event may be processed before the main UI thread tasks have finished. The keyboard processing thread may then cancel the UI work resulting in the UI not showing, which is good here

alt-tab-macos/logic/Keyboard.swift

@@ -22,21 +22,23 @@ class Keyboard {
 var eventTap: CFMachPort?
 
 func listenToGlobalKeyboardEvents(_ delegate: Application) {
-    let eventMask = [CGEventType.keyDown, CGEventType.keyUp, CGEventType.flagsChanged].reduce(CGEventMask(0), { $0 | (1 << $1.rawValue) })
-    eventTap = CGEvent.tapCreate(
-            tap: .cgSessionEventTap,
-            place: .headInsertEventTap,
-            options: .defaultTap,
-            eventsOfInterest: CGEventMask(eventMask),
-            callback: { (_, _, event, delegate_) -> Unmanaged<CGEvent>? in
-                let d = Unmanaged<Application>.fromOpaque(delegate_!).takeUnretainedValue()
-                return keyboardHandler(event, d)
-            },
-            userInfo: UnsafeMutableRawPointer(Unmanaged.passUnretained(delegate).toOpaque()))
-    let runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0)
-    CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, .commonModes)
-    CGEvent.tapEnable(tap: eventTap!, enable: true)
-    CFRunLoopRun()
+    DispatchQueue.global(qos: .userInteractive).async {
+        let eventMask = [CGEventType.keyDown, CGEventType.keyUp, CGEventType.flagsChanged].reduce(CGEventMask(0), { $0 | (1 << $1.rawValue) })
+        eventTap = CGEvent.tapCreate(
+                tap: .cgSessionEventTap,
+                place: .headInsertEventTap,
+                options: .defaultTap,
+                eventsOfInterest: CGEventMask(eventMask),
+                callback: { (_, _, event, delegate_) -> Unmanaged<CGEvent>? in
+                    let d = Unmanaged<Application>.fromOpaque(delegate_!).takeUnretainedValue()
+                    return keyboardHandler(event, d)
+                },
+                userInfo: UnsafeMutableRawPointer(Unmanaged.passUnretained(delegate).toOpaque()))
+        let runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0)
+        CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, .commonModes)
+        CGEvent.tapEnable(tap: eventTap!, enable: true)
+        CFRunLoopRun()
+    }
 }
 
 func keyboardHandler(_ cgEvent: CGEvent, _ delegate: Application) -> Unmanaged<CGEvent>? {

alt-tab-macos/ui/Application.swift

@@ -47,7 +47,7 @@ class Application: NSApplication, NSApplicationDelegate, NSWindowDelegate {
 
     func hideUi() {
         debugPrint("hideUi")
-        thumbnailsPanel!.orderOut(nil)
+        DispatchQueue.main.async(execute: { self.thumbnailsPanel!.orderOut(nil) })
         appIsBeingUsed = false
         isFirstSummon = true
     }
@@ -96,7 +96,7 @@ class Application: NSApplication, NSApplicationDelegate, NSWindowDelegate {
 
     func cycleSelection(_ step: Int) {
         selectedOpenWindow = cellWithStep(step)
-        thumbnailsPanel!.highlightCellAt(step)
+        DispatchQueue.main.async(execute: { self.thumbnailsPanel!.highlightCellAt(step) })
     }
 
     func showUiOrCycleSelection(_ step: Int) {
@@ -117,7 +117,6 @@ class Application: NSApplication, NSApplicationDelegate, NSWindowDelegate {
             }
             workItems.append(workItem)
             DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Preferences.windowDisplayDelay!, execute: workItem)
-
         } else {
             cycleSelection(step)
         }