[6.40] [gpad] Fix crash in streaming web canvas after TColor::DefinedColors(1)#22609
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guitargeek merged 1 commit intoJun 15, 2026
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TColor::DefinedColors(1) sets the global color bookkeeping to the sticky "always store colors" mode (gLastDefinedColors = -1). When a canvas is then serialized to JSON for the web display - as JupyROOT does to show a canvas inline in a notebook - TWebCanvas::CreatePadSnapshot temporarily detaches the pad primitives (fPrimitives = nullptr) and calls the no-arg TColor::DefinedColors() to clear the change flag so the canvas streamer skips color storage. In sticky mode that call returns kTRUE early without resetting the flag, so TCanvas::Streamer entered the color-storing branch and dereferenced the null fPrimitives, crashing with a segmentation violation. Guard the color-storing branch with a null check on fPrimitives. Colors and the palette are delivered separately by TWebCanvas::AddColorsPalette in that path, and normal file saving (where fPrimitives is never null) is unaffected. Add a regression test that reproduces the original report by drawing a canvas and calling TWebCanvas::CreateCanvasJSON after TColor::DefinedColors(1); it segfaults without this fix and passes with it. The JupyROOT notebook test is registered without output comparison (it draws a canvas, so its output is not reproducible), matching Cpp_IMT_Canvas.ipynb. Closes root-project#20018. 🤖 Done with the help of [Claude Code](https://claude.com/claude-code) (Claude Opus 4.8) (cherry picked from commit 2266020)
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Test Results 21 files 21 suites 3d 7h 10m 34s ⏱️ For more details on these failures, see this check. Results for commit 9f3e6b5. |
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Backport of #22603, requested by @guitargeek.