Eliminate layout shift in menu toggle arrow with CSS-only solution #1935
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Problem
Previously, the menu toggle arrow in the Table of Contents was implemented by updating the button’s inner HTML with different Unicode arrow entities (e.g., ► for closed and ▼ for open) via JavaScript. This approach caused a layout shift when toggling the menu, because the two Unicode arrow characters have different widths and font rendering. As a result, the button size and surrounding layout would visibly jump when the menu was opened or closed, negatively impacting user experience and visual stability.
Solution
This PR refactors the arrow indicator to use a CSS-only solution. The arrow is now rendered using a CSS ::after pseudo-element on the toggle button, with its direction controlled by toggling a class and applying a transform: rotate() property. This ensures the arrow always occupies the same space, eliminating layout shifts and providing a smoother, more consistent UI. The button’s HTML remains static, and all visual changes are handled via CSS transitions.
Mobile screens specific issue