linuxsessionmgr: make browser detection more robust

Now that we know how to launch a browser via $BROWSER (if set), we
should adjust our `IsWebBrowserAvailable` check for Linux.

In both WSL and non-WSL (native Linux) environments we immediately
return true when $BROWSER is set. VSCode remote tunnels set the $BROWSER
variable to a script that redirects requests back to the client's
browser, for example.

In non-WSL environments we also make our checks for a browser more
robust by checking for a 'shell execute' handler (like xdg-open).
Previously we just relied on a desktop session ($DISPLAY or
$WAYLAND_DISPLAY), which isn't accurate since in
BrowserUtils::OpenDefaultBrowser we require a shell execute handler (or
$BROWSER). The check and implementation are now aligned!

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

src/shared/Core/Interop/Linux/LinuxSessionManager.cs

@@ -21,16 +21,22 @@ public override bool IsWebBrowserAvailable
 
     private bool GetWebBrowserAvailable()
     {
+        // Check if the $BROWSER environment variable is set - we've been told explicitly we have one!
+        if (BrowserUtils.TryGetBrowserEnvironment(Environment, out _))
+        {
+            return true;
+        }
+
+        // We need a shell execute handler to be able to launch to browser
+        if (!BrowserUtils.TryGetLinuxShellExecuteHandler(Environment, out _))
+        {
+            return false;
+        }
+
         // If this is a Windows Subsystem for Linux distribution we may
         // be able to launch the web browser of the host Windows OS.
         if (WslUtils.IsWslDistribution(Environment, FileSystem, out _))
         {
-            // We need a shell execute handler to be able to launch to browser
-            if (!BrowserUtils.TryGetLinuxShellExecuteHandler(Environment, out _))
-            {
-                return false;
-            }
-
             //
             // If we are in Windows logon session 0 then the user can never interact,
             // even in the WinSta0 window station. This is typical when SSH-ing into a
@@ -55,10 +61,9 @@ private bool GetWebBrowserAvailable()
 
             // If we are not in session 0, or we cannot get the Windows session ID,
             // assume that we *CAN* launch the browser so that users are never blocked.
-            return true;
+            // Fall through to return true.
         }
 
-        // We require an interactive desktop session to be able to launch a browser
-        return IsDesktopSession;
+        return true;
     }
 }