wxScrolledWindow not working as expected - wxwidgets

Question

I have a problem with wxScrolledWindow.
I have a ScrolledWindow holding a wxListBox.
Behavior that works:
On adding items to the listbox the scrollbar will eventually appear.
Behavior that fails:
wxListBox has some items. When i reduce the size of the ScrolledWindow such that some items are occluded, the scrollbar fails to appear.
// split window into 2
wxBoxSizer *sizermain = new wxBoxSizer(wxVERTICAL);
wxSplitterWindow *splittermain = new wxSplitterWindow(this, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxSP_LIVE_UPDATE);
splittermain->SetSashGravity(0.5);
splittermain->SetMinimumPaneSize(20);
sizermain->Add(splittermain, 1,wxEXPAND,0);
this->SetSizer(sizermain);
// split left window into 2
wxSplitterWindow * splitterleft = new wxSplitterWindow(splittermain, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxSP_LIVE_UPDATE);
splitterleft->SetSashGravity(0.5);
splitterleft->SetMinimumPaneSize(20);
wxBoxSizer * sizerleft = new wxBoxSizer(wxHORIZONTAL);
sizerleft->Add(splitterleft, 1, wxEXPAND, 0);
// split right window into 2
wxSplitterWindow * splitteright = new wxSplitterWindow(splittermain, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxSP_LIVE_UPDATE);
splitteright->SetSashGravity(0.5);
splitteright->SetMinimumPaneSize(20);
wxBoxSizer * sizerright = new wxBoxSizer(wxHORIZONTAL);
sizerright->Add(splitteright, 1, wxEXPAND, 0);
wxScrolledWindow *tlpnl = new wxScrolledWindow(splitterleft, wxID_ANY);
wxListBox *tlcalls = new wxListBox(tlpnl, wxID_ANY, wxDefaultPosition, wxDefaultSize, 0, nullptr, wxLB_EXTENDED);
wxBoxSizer *tlsizer = new wxBoxSizer(wxVERTICAL);
tlsizer->Add(tlcalls, 1,wxEXPAND,0);
tlpnl->SetSizer(tlsizer);
wxScrolledWindow *blpnl = new wxScrolledWindow(splitterleft, wxID_ANY);
wxListBox *blcontacts = new wxListBox(blpnl, wxID_ANY, wxDefaultPosition, wxDefaultSize, 0, nullptr, wxLB_EXTENDED);
wxBoxSizer *blsizer = new wxBoxSizer(wxVERTICAL);
blsizer->Add(blcontacts, 1,wxEXPAND,0);
blpnl->SetSizer(blsizer);
wxScrolledWindow *trpnl = new wxScrolledWindow(splitteright, wxID_ANY);
inCall = new wxListBox(trpnl, wxID_ANY, wxDefaultPosition, wxDefaultSize, 0, nullptr, wxLB_EXTENDED);
wxBoxSizer *trsizer = new wxBoxSizer(wxVERTICAL);
trsizer->Add(inCall, 1,wxEXPAND,0);
trpnl->SetSizer(trsizer);
wxScrolledWindow *brpnl = new wxScrolledWindow(splitteright, wxID_ANY);
wxTextCtrl *brtext = new wxTextCtrl(brpnl, wxID_ANY);
wxBoxSizer *brsizer = new wxBoxSizer(wxVERTICAL);
brsizer->Add(brtext, 1,wxEXPAND,0);
brpnl->SetSizer(brsizer);
splittermain->SplitVertically(splitterleft, splitteright);
splitterleft->SplitHorizontally(tlpnl, blpnl);
splitteright->SplitHorizontally(trpnl, brpnl);

Answer 1

Your code doesn't seem to show the problem (as there are no items anywhere) but there is at least one thing wrong with it: you create sizerLeft and sizerRight sizers and add something to them but never use these sizers themselves. This might explain the layout problems... But if it doesn't, you really should show the code that actually shows the problem you're describing (and perhaps shorter than what you have, are both splitters actually needed?).