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875ca70a55
screen_draw_overlay_line accessed self->linebuf->line->cpu_cells without ever calling linebuf_init_line on the shared view. Render paths that initialize a stack-local Line via render_line_for_virtual_y left the view's cpu_cells as NULL (the value set by alloc_line via PyType_GenericAlloc), and the multicell-trim loop then dereferenced NULL + xstart * sizeof(CPUCell), producing a SIGSEGV at a small address (e.g. 0x1e for xstart=2). The crash was reachable any time an IME pre-edit overlay was rendered with the cursor not in column 0 on a screen whose linebuf->line had not been re-pointed by some unrelated prior call. Fix by initializing the view at the overlay row on entry. Add a test_draw_overlay_line method on Screen so the behavior can be exercised directly from a regression test.