[Lock Screen] Do not try to unlock when unvisible

Summary:
In the lock screen, pressing the enter key while the UI is not yet
visible ends up trying to unlock (and would usually fail because the
password field is empty).
Ensure that the UI is visible before unlocking.

BUG: 395671

Test Plan:
Lock the session, press enter. The UI should appear without any
"Unlocking failed" message.

Reviewers: #plasma, #vdg, ngraham, davidedmundson

Reviewed By: #plasma, #vdg, ngraham, davidedmundson

Subscribers: ngraham, davidedmundson, abetts, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16219

lookandfeel/contents/lockscreen/LockScreenUi.qml

@@ -211,6 +211,7 @@ PlasmaCore.ColorScope {
 
             initialItem: MainBlock {
                 id: mainBlock
+                lockScreenUiVisible: lockScreenRoot.uiVisible
 
                 showUserList: userList.y + mainStack.y > 0
 

lookandfeel/contents/lockscreen/MainBlock.qml

@@ -30,6 +30,7 @@ import "../components"
 SessionManagementScreen {
 
     property Item mainPasswordBox: passwordBox
+    property bool lockScreenUiVisible: false
 
     //the y position that should be ensured visible when the on screen keyboard is visible
     property int visibleBoundary: mapFromItem(loginButton, 0, 0).y
@@ -61,7 +62,11 @@ SessionManagementScreen {
         enabled: !authenticator.graceLocked
         revealPasswordButtonShown: true
 
-        onAccepted: startLogin()
+        onAccepted: {
+            if (lockScreenUiVisible) {
+                startLogin();
+            }
+        }
 
         //if empty and left or right is pressed change selection in user switch
         //this cannot be in keys.onLeftPressed as then it doesn't reach the password box