Added documentation for non-opaque undocking

Author: githubuser0xFFFF

README.md

@@ -69,9 +69,18 @@ main window layout.
 ![Perspective](doc/perspectives_dark.png)
 
 ### Opaque and non-opaque splitter resizing
-The advanced docking system uses standard QSplitters as resize separators and thus supports opaque and non-opaque resizing functionality of QSplitter. In some rare cases, for very complex widgets or on slow machines resizing via separator on the fly may cause flicking and glaring of rendered content inside a widget. The global CDockManager flag `OpaqueSplitterResize` configures the resizing behaviour of the splitters. If this flag is set, then widgets are resized dynamically (opaquely) while interactively moving the splitters. If this flag is cleared, the widget resizing is deferred until the mouse button is released - this is some kind of lazy resizing separator.
+The advanced docking system uses standard QSplitters as resize separators and thus supports opaque and non-opaque resizing functionality of QSplitter. In some rare cases, for very complex widgets or on slow machines resizing via separator on the fly may cause flicking and glaring of rendered content inside a widget. The global dock manager flag `OpaqueSplitterResize` configures the resizing behaviour of the splitters. If this flag is set, then widgets are resized dynamically (opaquely) while interactively moving the splitters. If this flag is cleared, the widget resizing is deferred until the mouse button is released - this is some kind of lazy resizing separator.
 
 ### Opaque and non-opaque undocking
+By default, opaque undocking is active. That means, as soon as you drag a dock widget or a dock area with a number of dock widgets it will be undocked and moved into a floating widget and then the floating widget will be dragged around. That means undocking will take place immediatelly. You can compare this with opaque splitter resizing. If the flag `OpaqueUndocking` is cleared, then non-opaque undocking is active. In this mode, undocking is more like a standard drag and drop operation. That means, the dragged dock widget or dock area is not undocked immediatelly. Instead, a drag preview widget is created and dragged around to indicate the future position of the dock widget or dock area. The actual dock operation is only executed when the mouse button is released. That makes it possible, to cancel an active drag operation with the escape key.
+
+The drag preview widget can be configured by a number of global dock manager flags:
+- `DragPreviewIsDynamic`: if this flag is enabled, the preview will be adjusted dynamically to the drop area
+- `DragPreviewShowsContentPixmap`: the created drag preview window shows a static copy of the content of the dock widget / dock are that is dragged
+- `DragPreviewHasWindowFrame`: this flag configures if the drag preview is frameless like a QRubberBand or looks like a real window
+
+The best way to test non-opaque undocking is to set the standard flags: `CDockManager::setConfigFlags(CDockManager::DefaultNonOpaqueConfig)`.
+
 ### Tab-menu for easy handling of many tabbed dock widgets
 ### Many different ways to detach dock widgets
 ### Supports deletion of dynamically created dock widgets