Really fix wxProcessDialog size when the message length changes

Ensure that we only increase the dialog width but never decrease it to
prevent the dialog from jumping annoyingly to different size with each
label change.

The previous attempt to do this in 4101849b4d (Grow
wxGenericProgressDialog automatically if needed, 2017-10-27) didn't work
because it seemingly didn't take into account the fact that wxStaticText
width changes when its label changes and so calling SetLabel() could
make it smaller, meaning that the next update would call Fit() again.

So fix this by preventing the control from changing its size by using
wxST_NO_AUTORESIZE and then only resizing it manually if it really needs
more space.

See #10624, #23727.

(cherry picked from commit 97546a6f0cf941ce94b42a27b23f51e224c8e54e)

docs/changes.txt

@@ -264,6 +264,7 @@ All (GUI):
 - Relax checks on character entry in numeric validators (#12968, #23561).
 - Fix wxRibbonToolBar::GetToolByPos() and DeleteToolByPos() (mcorino, #23457).
 - Update wxOwnerDrawnComboBox best size when adding items (Reza Karimi, #23681).
+- Keep wxProgressDialog size when updating the message (Oleg Samarin, #23727).
 
 wxGTK:
 

src/generic/progdlgg.cpp

@@ -179,7 +179,14 @@ bool wxGenericProgressDialog::Create( const wxString& title,
     // top-level sizerTop
     wxSizer * const sizerTop = new wxBoxSizer(wxVERTICAL);
 
-    m_msg = new wxStaticText(this, wxID_ANY, message);
+    // We use wxST_NO_AUTORESIZE to prevent the label from snapping back to
+    // smaller size if the message becomes shorter: we need this because we
+    // always increase its size to fit the longest message and so we assume
+    // that its current size is always this longest size and not some maybe
+    // shorter size.
+    m_msg = new wxStaticText(this, wxID_ANY, message,
+                             wxDefaultPosition, wxDefaultSize,
+                             wxST_NO_AUTORESIZE);
     sizerTop->Add(m_msg, 0, wxLEFT | wxRIGHT | wxTOP, 2*LAYOUT_MARGIN);
 
     int gauge_style = wxGA_HORIZONTAL;
@@ -787,12 +794,18 @@ void wxGenericProgressDialog::UpdateMessage(const wxString &newmsg)
 {
     if ( !newmsg.empty() && newmsg != m_msg->GetLabel() )
     {
-        const wxSize sizeOld = m_msg->GetSize();
-
         m_msg->SetLabel(newmsg);
 
-        if ( m_msg->GetSize().x > sizeOld.x )
+        // When using wxST_NO_AUTORESIZE, best size is not invalidated by
+        // changing the label, but we do want to compute the best size here
+        // so do it manually.
+        m_msg->InvalidateBestSize();
+
+        const wxSize sizeNeeded = m_msg->GetBestSize();
+        if ( sizeNeeded.x > m_msg->GetSize().x )
         {
+            m_msg->SetSize(sizeNeeded);
+
             // Resize the dialog to fit its new, longer contents instead of
             // just truncating it.
             Fit();