FIX(client): Improve handling of the GlobalShortcutButtons dialog

This commit makes the shortcut configuration dialog a modal window.
It also explicitly searches for a new effective parent, such that the window
can be centered over the Mumble application instead of randomly out of bounds
of the screen. (This is most likely only necessary because of a bug in Qt, but
Qt is a pain to debug)

Fixes mumble-voip#5024

src/mumble/GlobalShortcutButtons.cpp

@@ -8,15 +8,42 @@
 
 #include "GlobalShortcut.h"
 
+#include "MainWindow.h"
 #include "Global.h"
 
+#include <QRect>
 #include <QTimer>
 
 GlobalShortcutButtons::GlobalShortcutButtons(QWidget *parent) : QDialog(parent), m_ui(new Ui::GlobalShortcutButtons) {
 	m_ui->setupUi(this);
+	setModal(true);
 
 	connect(m_ui->addButton, &QPushButton::toggled, this, &GlobalShortcutButtons::toggleCapture);
 	connect(m_ui->removeButton, &QPushButton::clicked, this, &GlobalShortcutButtons::removeItem);
+
+	// Due to what appears to be a Qt bug, this dialog will not take
+	// the parent geometry into account when spawning.
+	// Therefore, we search for a different parent or even fallback to the
+	// MainWindow and move this dialog to its center.
+	QWidget *searchWidget = parent;
+	while (searchWidget && !qobject_cast< QDialog * >(searchWidget)) {
+		searchWidget = searchWidget->parentWidget();
+	}
+	QWidget *effectiveParent = qobject_cast< QDialog * >(searchWidget);
+	if (!effectiveParent && Global::get().mw && Global::get().mw->isVisible()) {
+		effectiveParent = Global::get().mw;
+	}
+
+	int32_t posx = geometry().x();
+	int32_t posy = geometry().y();
+
+	if (effectiveParent) {
+		QRect geometry = effectiveParent->geometry();
+		posx           = geometry.center().x() - (width() / 2);
+		posy           = geometry.center().y() - (height() / 2);
+	}
+
+	QTimer::singleShot(0, [this, posx, posy]() { move(posx, posy); });
 }
 
 GlobalShortcutButtons::~GlobalShortcutButtons() {