Linux: Fix dragging of maximized floating windows

They should restore their normal size when the drag starts

(cherry-picked from commit 1305dee081)

src/controllers/TitleBar.cpp

@@ -19,6 +19,7 @@
 
 #include "views/TitleBarViewInterface.h"
 #include "ViewWrapper.h"
+#include "controllers/DockWidget_p.h"
 #include "controllers/FloatingWindow.h"
 #include "controllers/TabBar.h"
 #include "controllers/MainWindow.h"
@@ -371,10 +372,22 @@ void TitleBar::onFloatClicked()
                     return;
                 }
 
+                int i = 0;
+                DockWidget *current = nullptr;
                 for (auto dock : qAsConst(dockWidgets)) {
+
+                    if (!current && dock->isCurrentTab())
+                        current = dock;
+
                     dock->setFloating(true);
+                    dock->dptr()->m_lastPosition->m_tabIndex = i;
                     dock->setFloating(false);
+                    ++i;
                 }
+
+                // Restore the current tab
+                if (current)
+                    current->setAsCurrentTab();
             }
         }
     } else {

src/private/DragController.cpp

@@ -317,7 +317,38 @@ void StateDragging::onEntry()
                        << "; m_windowBeingDragged=" << q->m_windowBeingDragged->floatingWindow();
 
         auto fw = q->m_windowBeingDragged->floatingWindow();
-        if (!fw->geometry().contains(q->m_pressPos)) {
+#ifdef Q_OS_LINUX
+        if (fw->view()->isMaximized()) {
+            // When dragging a maximized window on linux we need to restore its normal size
+            // On Windows this works already. On macOS I don't see this feature at all
+
+            const QRect normalGeometry = fw->normalGeometry();
+
+            // distance to the left edge of the window:
+            const int leftOffset = q->m_offset.x();
+
+            // distance to the right edge of the window:
+            const int rightOffset = fw->width() - q->m_offset.x();
+
+            const bool leftEdgeIsNearest = leftOffset <= rightOffset;
+
+            fw->showNormal();
+
+            if (!normalGeometry.contains(q->m_pressPos)) {
+                if ((leftEdgeIsNearest && leftOffset > normalGeometry.width()) || (!leftEdgeIsNearest && rightOffset > normalGeometry.width())) {
+                    // Case #1: The window isn't under the cursor anymore
+                    // Let's just put its middle under the cursor
+                    q->m_offset.setX(normalGeometry.width() / 2);
+                } else if (!leftEdgeIsNearest) {
+                    // Case #2: The new geometry is still under the cursor, but instead of moving its right edge left
+                    // we'll move the left edge right, since initially the press position was closer to the right edge
+                    q->m_offset.setX(normalGeometry.width() - rightOffset);
+                }
+            }
+        } else
+#endif
+
+            if (!fw->geometry().contains(q->m_pressPos)) {
             // The window shrunk when the drag started, this can happen if it has max-size constraints
             // we make the floating window smaller. Has the downside that it might not be under the mouse
             // cursor anymore, so make the change