Fix code editor tab width after horizontal scroll

[RDeckard]

Why

Original issue: #2940

Follow-up to #2901

The previous tab-aware mouse mapping fix (#2901) handled visible tabs, but #2940 shows a remaining mismatch after horizontal scrolling hides indentation tabs. In that case, hidden tabs could stop advancing the rendered x position by their full visual width, while mouse mapping still used visual tab columns.

What

• Reuse the existing tab-column width calculation for code rendering.
• Compute a tab's x advance before the visibility check, so offscreen tabs still affect later character positions.
• Make drawTab(...) draw using the already computed visual width instead of recalculating from screen x.

Impact

This should make rendering and mouse hit testing agree better when tabs are scrolled offscreen horizontally. I have not manually reproduced the exact video case, so reviewer confirmation on the #2940 scenario would be useful.