[gnome-shell] ControlsManagerLayout: Use the workarea size to compute the available height

[Marge Bot <marge-bot src gnome org>]

commit f164e086880860a126fde6b6bfa267d8ecbab414
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Fri Jun 18 19:28:55 2021 +0200

    ControlsManagerLayout: Use the workarea size to compute the available height
    
    To compute the available height for the layout we're currently using the
    panel position, while this works for the current and default setup, the
    shell may be configured to use a different workarea, so we should rely on
    it to compute the available space, instead of a specific widget.
    
    So get the current monitor index for the current view and use its coordinates
    instead.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1892>

 js/ui/overviewControls.js | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)
---
diff --git a/js/ui/overviewControls.js b/js/ui/overviewControls.js
index c153161696..df098ace95 100644
--- a/js/ui/overviewControls.js
+++ b/js/ui/overviewControls.js
@@ -131,11 +131,10 @@ class ControlsManagerLayout extends Clutter.BoxLayout {
 
         const { spacing } = this;
 
-        let startY = 0;
-        if (Main.layoutManager.panelBox.y === Main.layoutManager.primaryMonitor.y) {
-            startY = Main.layoutManager.panelBox.height;
-            box.y1 += startY;
-        }
+        const monitor = Main.layoutManager.findMonitorForActor(this._container);
+        const workArea = Main.layoutManager.getWorkAreaForMonitor(monitor.index);
+        const startY = workArea.y - monitor.y;
+        box.y1 += startY;
         const [width, height] = box.get_size();
         let availableHeight = height;