Do not update WindowTitle at all in prompt func if setting is $null

We used to restore the previousWindowTitle but this had the effect of
always updating the WindowTitle with a fixed string - whatever we
captured in $PreviousWindowTitle.

However, we still restore the original window title when the module is
removed. But we only do this if the module is configured to supply the
window title.

Rename variable to make its intent clearer.

Fixes #594

src/posh-git.psm1

@@ -18,16 +18,17 @@ if (!$Env:HOME) { $Env:HOME = "$Env:USERPROFILE" }
 $IsAdmin = Test-Administrator
 
 # Probe $Host.UI.RawUI.WindowTitle to see if it can be set without errors
-$WindowTitleSupported = $false
+$HostSupportsSettingWindowTitle = $false
 try {
-    $global:PreviousWindowTitle = $Host.UI.RawUI.WindowTitle
-    $newTitle = "${global:PreviousWindowTitle} "
+    $global:PoshGitOrigWindowTitle = $Host.UI.RawUI.WindowTitle
+    $newTitle = "${global:PoshGitOrigWindowTitle} "
     $Host.UI.RawUI.WindowTitle = $newTitle
-    $WindowTitleSupported = ($Host.UI.RawUI.WindowTitle -eq $newTitle)
-    $Host.UI.RawUI.WindowTitle = $global:PreviousWindowTitle
+    $HostSupportsSettingWindowTitle = ($Host.UI.RawUI.WindowTitle -eq $newTitle)
+    $Host.UI.RawUI.WindowTitle = $global:PoshGitOrigWindowTitle
 }
 catch {
-    Write-Debug "Probing for WindowTitleSupported errored: $_"
+    $global:PoshGitOrigWindowTitle = $null
+    Write-Debug "Probing for HostSupportsSettingWindowTitle errored: $_"
 }
 
 # Get the default prompt definition.
@@ -43,10 +44,6 @@ else {
 $GitPromptScriptBlock = {
     $settings = $global:GitPromptSettings
     if (!$settings) {
-        if ($WindowTitleSupported -and $global:PreviousWindowTitle) {
-            $Host.UI.RawUI.WindowTitle = $global:PreviousWindowTitle
-        }
-
         return "<`$GitPromptSettings not found> "
     }
 
@@ -92,34 +89,22 @@ $GitPromptScriptBlock = {
         $promptSuffix.Text = $promptSuffix.Text.Substring(0, $promptSuffix.Text.Length - 1)
     }
 
-    # Update the host's WindowTitle is host supports it and user has not disabled $GitPromptSettings.WindowTitle
+    # Update the host's WindowTitle if host supports it and user has not disabled $GitPromptSettings.WindowTitle
     # This has to be *after* the call to Write-VcsStatus, which populates $global:GitStatus
-    if ($WindowTitleSupported) {
-        $windowTitle = $settings.WindowTitle
-        if (!$windowTitle) {
-            if ($global:PreviousWindowTitle) {
-                $Host.UI.RawUI.WindowTitle = $global:PreviousWindowTitle
+    if ($HostSupportsSettingWindowTitle -and $settings.WindowTitle) {
+        try {
+            if ($settings.WindowTitle -is [scriptblock]) {
+                $windowTitleText = & $settings.WindowTitle $global:GitStatus $IsAdmin
             }
-        }
-        else {
-            try {
-                if ($windowTitle -is [scriptblock]) {
-                    $windowTitleText = & $windowTitle $global:GitStatus $IsAdmin
-                }
-                else {
-                    $windowTitleText = $ExecutionContext.SessionState.InvokeCommand.ExpandString("$windowTitle")
-                }
-
-                # Put $windowTitleText in a string to ensure results returned by scriptblock are flattened to a string
-                $Host.UI.RawUI.WindowTitle = "$windowTitleText"
+            else {
+                $windowTitleText = $ExecutionContext.SessionState.InvokeCommand.ExpandString("$($settings.WindowTitle)")
             }
-            catch {
-                if ($global:PreviousWindowTitle) {
-                    $Host.UI.RawUI.WindowTitle = $global:PreviousWindowTitle
-                }
 
-                Write-Debug "Error occurred during evaluation of `$GitPromptSettings.WindowTitle: $_"
-            }
+            # Put $windowTitleText in a string to ensure results returned by scriptblock are flattened into a string
+            $Host.UI.RawUI.WindowTitle = "$windowTitleText"
+        }
+        catch {
+            Write-Debug "Error occurred during evaluation of `$GitPromptSettings.WindowTitle: $_"
         }
     }
 
@@ -171,9 +156,9 @@ if ($ForcePoshGitPrompt -or !$currentPromptDef -or ($currentPromptDef -eq $defau
 $ExecutionContext.SessionState.Module.OnRemove = {
     $global:VcsPromptStatuses = $global:VcsPromptStatuses | Where-Object { $_ -ne $PoshGitVcsPrompt }
 
-    # Revert original WindowTitle
-    if ($WindowTitleSupported -and $global:PreviousWindowTitle) {
-        $Host.UI.RawUI.WindowTitle = $global:PreviousWindowTitle
+    # Revert original WindowTitle but only if posh-git is currently configured to set it
+    if ($HostSupportsSettingWindowTitle -and $global:GitPromptSettings.WindowTitle -and $global:PoshGitOrigWindowTitle) {
+        $Host.UI.RawUI.WindowTitle = $global:PoshGitOrigWindowTitle
     }
 
     # Check if the posh-git prompt function itself has been replaced. If so, do not restore the prompt function