Added notes about the sizer changes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@28266 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -18,7 +18,91 @@ INCOMPATIBLE CHANGES SINCE 2.4.x
   exists but doesn't do anything any more, this behaviour is default now)
 
 ! wxWindow::m_font and m_backgroundColour/m_foregroundColour are no longer
-  always set, use GetFont(), GetBack/ForegroundColour() to access them
+  always set, use GetFont(), GetBack/ForegroundColour() to access
+  them, and they will be dynamically determined if necessary.
+
+
+! The Sizers have had some fundamental internal changes in the 2.5.2
+  and 2.5.3 releases intended to make them do more of the "Right
+  Thing" but also be as backwards compatible as possible.  First a bit
+  about how things used to work:
+
+    * The size that a window had when Add()ed to the sizer was assumed
+      to be its minimal size, and that size would always be used by
+      default when calculating layout size and positions, and the
+      sizer itself would keep track of that minimal size.
+
+    * If the window item was Add()ed with the wxADJUST_MINSIZE flag
+      then when layout was calculated the item's GetBestSize would be
+      used to reset the minimal size that the sizer used.
+
+  The main thrust of the new Sizer changes was to make behaviour like
+  wxADJUST_MINSIZE be the default, and also to push the tracking of
+  the minimal size to the window itself (since it knows its own needs)
+  instead of having the sizer take care of it.  Consequently these
+  changes were made:
+
+    * The wxFIXED_MINSIZE flag was added to allow for the old
+      behaviour.  When this flag is used the size a window has when
+      Add()ed to the sizer will be treated as its minimal size and it
+      will not be readjusted on each layout.
+
+    * The min size stored in wxWindow and settable with SetSizeHints or
+      SetMinSize will by default be used by the sizer (if it was set)
+      as the minimal size of the sizer item.  If the minsize was not
+      set (or was only partially set) then the window's best size is
+      fetched and it is used instead of (or blended with) the minsize.
+      wxWindow:GetBestFittingSize was added to facilitate getting the
+      size to be used by the sizers.
+
+    * The best size of a window is cached so it doesn't need to
+      recaculated on every layout.  wxWindow::InvalidateBestSize was
+      added and should be called (usually just internally in control
+      methods) whenever something is done that would make the best
+      size change.
+
+    * All wxControls were changed to set the minsize to what is passed
+      to the constructor or Create method, and also to set the real
+      size of the control to the blending of the minsize and bestsize.
+      wxWindow::SetBestFittingSize was added to help with this,
+      although most controls don't need to call it directly because it
+      is called indirectly via the SetInitialSize called in the base
+      classes.
+
+  At this time, the only situation known not to work the same as
+  before is the following:
+
+	 win = new SomeWidget(parent);
+	 win->SetSize(SomeNonDefaultSize);
+	 sizer->Add(win);
+
+  In this case the old code would have used the new size as the
+  minimum, but now the sizer will use the default size as the minimum
+  rather than the size set later.  It is an easy fix though, just move
+  the specification of the size to the constructor (assuming that
+  SomeWidget will set its minsize there like the rest of the controls
+  do) or call SetMinSize instead of SetSize.
+
+  In order to fit well with this new scheme of things, all wxControls
+  or custom controls should do the following things.  (Depending on
+  how they are used you may also want to do the same thing for
+  non-control custom windows.)
+
+    * Either override or inherit a meaningful DoGetBestSize method
+      that calculates whatever size is "best" for the control.  Once
+      that size is calculated then there should normally be a call to
+      CacheBestSize to save it for later use, unless for some reason
+      you want the best size to be recalculated on every layout.
+
+    * Any method that changes the attributes of the control such that
+      the best size will change should call InvalidateBestSize so it
+      will be recalculated the next time it is needed.
+
+    * The control's constructor and/or Create method should ensure
+      that the minsize is set to the size passed in, and that the
+      control is sized to a blending of the min size and best size.
+      This can be done by calling SetBestFittingSize.
+
 
 
 - no initialization/cleanup can be done in wxApp/~wxApp because they are