Fix `ScrollBar` grabber mouse input ignores scroll content margins. #41629

Job	Run time
📊 Static checks / Code style, file formatting, and docs	42s
🍎 macOS / Editor (target=editor, tests=yes)	27m 36s
🤖 Android / Editor (target=editor)	5m 46s
🍎 macOS / Template (target=template_release, tests=yes)	4m 46s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 37s
🍏 iOS / Template (target=template_release)	2m 2s
🐧 Linux / Editor w/ Mono (target=editor)	7m 17s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	17m 45s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 54s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 47s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	8m 18s
🌐 Web / Template w/o threads (target=template_release, threads=no)	7m 32s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	8m 21s
🐧 Linux / Template w/ Mono (target=template_release, tests=yes)	1m 52s
🏁 Windows / Editor (target=editor, tests=yes)	9m 3s
🐧 Linux / Minimal template (target=template_release, tests=yes, everything disabled)	2m 33s
🏁 Windows / Editor w/ clang-cl (target=editor, tests=yes, use_llvm=yes)	5m 29s
🏁 Windows / Template (target=template_release, tests=yes)	8m 31s
🪲 Godot CPP / Build and test Godot CPP	8m 25s
	2h 20m 16s

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