From 5d95a18b6b58a9e5c600a720b9481122b6b27f1a Mon Sep 17 00:00:00 2001
From: Anton Schwarz <anton.schwarz@cern.ch>
Date: Wed, 26 Aug 2026 15:40:24 +0200
Subject: [PATCH] [gui] Native GUI draws canvas without defaulting to web

When drawing a canvas in a native GUI (e.g. using
`tutorials/visualisation/gui/guitest.C` and Test->Dialog->Tab3), the canvas
used to default to black. The workaround was `--web=off`, which is
inconvenient.

Implements a new method `TGuiFactory::CreateCanvasImpNative` that bypasses the
`gROOT->IsWebDisplay()` check and directly returns a `TCanvasImp` as required
by the native GUI.

`TGuiFactory::CreateCanvasImp` receives `TCanvas *c` as a parameter. For a web
canvas, the canvas ID is `111222333` (see `TWebCanvas.cxx:293`), this would be
enough to determine that we do not want to create a web canvas element, but a
native one instead. However, `TCanvas` is only forward declared within
`TGuiFactory`, hence we cannot access its methods.
---
 core/gui/inc/TGuiFactory.h   | 1 +
 core/gui/src/TGuiFactory.cxx | 8 ++++++++
 graf2d/gpad/src/TCanvas.cxx  | 2 +-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/core/gui/inc/TGuiFactory.h b/core/gui/inc/TGuiFactory.h
index 86045fe65642e..6c91e564beb8a 100644
--- a/core/gui/inc/TGuiFactory.h
+++ b/core/gui/inc/TGuiFactory.h
@@ -50,6 +50,7 @@ class TGuiFactory : public TNamed {
    virtual TApplicationImp *CreateApplicationImp(const char *classname, int *argc, char **argv);
 
    virtual TCanvasImp *CreateCanvasImp(TCanvas *c, const char *title, UInt_t width, UInt_t height);
+   virtual TCanvasImp *CreateCanvasImpNative(TCanvas *c, const char *title, UInt_t width, UInt_t height);
    virtual TCanvasImp *CreateCanvasImp(TCanvas *c, const char *title, Int_t x, Int_t y, UInt_t width, UInt_t height);
 
    virtual TBrowserImp *CreateBrowserImp(TBrowser *b, const char *title, UInt_t width, UInt_t height, Option_t *opt="");
diff --git a/core/gui/src/TGuiFactory.cxx b/core/gui/src/TGuiFactory.cxx
index cacc618583bdc..b9302e744a55b 100644
--- a/core/gui/src/TGuiFactory.cxx
+++ b/core/gui/src/TGuiFactory.cxx
@@ -81,6 +81,14 @@ TCanvasImp *TGuiFactory::CreateCanvasImp(TCanvas *c, const char *title, UInt_t w
    return new TCanvasImp(c, title, width, height);
 }
 
+////////////////////////////////////////////////////////////////////////////////
+/// Create a batch version of TCanvasImpNative, skipping the IsWebDisplay check.
+
+TCanvasImp *TGuiFactory::CreateCanvasImpNative(TCanvas *c, const char *title, UInt_t width, UInt_t height)
+{
+   return new TCanvasImp(c, title, width, height);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Create a batch version of TCanvasImp.
 
diff --git a/graf2d/gpad/src/TCanvas.cxx b/graf2d/gpad/src/TCanvas.cxx
index ee2b1a0819f32..ecd80939fd852 100644
--- a/graf2d/gpad/src/TCanvas.cxx
+++ b/graf2d/gpad/src/TCanvas.cxx
@@ -255,7 +255,7 @@ TCanvas::TCanvas(const char *name, Int_t ww, Int_t wh, Int_t winid) : TPad(), fD
          fUseGL = kFALSE;
    }
 
-   fCanvasImp = gBatchGuiFactory->CreateCanvasImp(this, name, fCw, fCh);
+   fCanvasImp = gBatchGuiFactory->CreateCanvasImpNative(this, name, fCw, fCh);
    if (!fCanvasImp) return;
 
    CreatePainter();