Wiki panel layout: Content clipping and unresponsive at smaller breakpoints

Problem

The Wiki is experiencing layout issues when using the panel layout, specifically:

• Content is clipping/being cut off
• Layout is not responsive at smaller breakpoints

These issues occur in both GDK and GitLab.com (observed in personal projects).

Current Behavior

When viewing Wiki pages in the panel layout:

• Content gets clipped and is not fully visible
• The layout does not adapt properly at smaller screen sizes

Expected Behavior

• All Wiki content should be fully visible without clipping
• Layout should be responsive and adapt gracefully across all breakpoints (mobile: 320px-414px, tablet: 768px-1024px, desktop: 1280px+)
• Content should remain readable and accessible at all screen sizes

Steps to Reproduce

1. Navigate to a Wiki page
2. View the page using panel layout
3. Resize browser to smaller breakpoints
4. Observe content clipping and layout issues

Screenshots

Create / Edit mode

Actions menu

Details page with sidebar collapsed

Impact

• Usability issues for mobile and tablet users
• Inconsistent experience across breakpoints

Acceptance Criteria

• Wiki content displays fully without clipping at all breakpoints
  • Panel layout is responsive from 320px to 2560px+ widths
  • No horizontal scrolling or overflow issues
  • Tested on mobile (320px, 375px, 414px), tablet (768px, 1024px), and desktop (1280px, 1440px, 1920px, 2560px) breakpoints
  • Tested with Fluid and Fixed layouts
  • Tested with old UI on both modes: Fluid and Fixed layouts
  • Test risizable wiki sidebar on all modes

Additional Context

• Related to the panel layout implementation in Wiki.
• May need to review CSS and responsive design patterns used in other GitLab features (Issues, MRs) for consistency.