Fix SelectableLabel text shift with inactive strokes

[sjh9714]

• Closes Button::selectable (SelectableLabel) shifts content by inactive.bg_stroke.width px on hover when frame_when_inactive = false #8125
• I have followed the instructions in the PR template

Summary

Button::atom_ui used the stroke-compensated frame margin even when an inactive frameless selectable button did not paint that stroke. With a non-zero inactive bg stroke, the text jumped when hovering and the hover frame was painted.

This restores the unpainted stroke padding for the inactive/no-frame path only, and adds a kittest regression that compares the text shape position before and after hover.

Testing

• PATH="$HOME/.cargo/bin:$PATH" cargo test -p egui_kittest selectable_label_text_should_not_shift_when_inactive_stroke_is_unpainted -- --nocapture (failed before the fix with [11.0 9.5] vs [12.0 9.5], passes after)
• PATH="$HOME/.cargo/bin:$PATH" cargo fmt --check
• PATH="$HOME/.cargo/bin:$PATH" cargo test -p egui_kittest --test regression_tests (passes; two pre-existing warnings in override_text_color_affects_interactive_widgets)
• PATH="$HOME/.cargo/bin:$PATH" cargo test -p egui
• PATH="$HOME/.cargo/bin:$PATH" cargo clippy -p egui -- -D warnings
• PATH="$HOME/.cargo/bin:$PATH" cargo clippy -p egui -p egui_kittest --test regression_tests -- -D warnings (fails on existing clippy::unnecessary_wraps in crates/egui_kittest/src/app_kind.rs:26)
• git diff --check

I opened this as draft because I did not run ./scripts/check.sh.

Note: I used Codex while preparing this change, reviewed the final diff, and ran the listed checks locally.

[github-actions]

Preview is being built...

Preview will be available at https://egui-pr-preview.github.io/pr/8276-fix-selectable-label-stroke-jitter

View snapshot changes at kitdiff