[4.3] Fix `ScrollBar` grabber mouse ignores scroll margin #602

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 8s
🍏 iOS / Template (target=template_release)	10m 4s
🤖 Android / Editor (target=editor)	24m 31s
🐧 Linux / Editor w/ Mono (target=editor)	41m 8s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	17m 2s
🐧 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)	44m 3s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	18m 5s
🍎 macOS / Editor (target=editor, tests=yes)	30m 3s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	37m 3s
🍎 macOS / Template (target=template_release)	19m 12s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	32m 19s
🐧 Linux / Template w/ Mono (target=template_release)	27m 41s
🐧 Linux / Minimal template (target=template_release, everything disabled)	15m 3s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	25m 41s
🏁 Windows / Editor (target=editor, tests=yes)	48m 24s
🌐 Web / Template w/o threads (target=template_release, threads=no)	25m 48s
🏁 Windows / Template (target=template_release)	36m 58s
🪲 Godot CPP / Build and test Godot CPP	7m 53s
	7h 42m 6s

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