Feature: fake selection improvement

[ashiq-prospero]

Description

Current implementation of SelectionAlwaysOnDisplay is not good, infact its worst, only a background color is applied to the fake selection but if certain text is having background color then selection gets skipped (selection is not visible) for that part of text

Either it should be configurable with background color and text color or something better

[pro-vi]

Confirming this, with the concrete mechanism. markSelection's defaultOnReposition draws each rect with background: 'Highlight' and color: 'HighlightText', both hardcoded, and positionNodeOnRange prepends the rect wrapper to root.parentElement. So the rects sit behind the glyphs and below the editable in paint order. Any span with its own opaque background (inline code, a code block) paints over the behind-text rect, and the selection disappears for that span. The color: 'HighlightText' is also inert here, since the rect divs are empty boxes behind the text, so today there's effectively one knob and it's not configurable.

We hit the same wall building a custom-drawn selection for #7106. Lifting the rect above the content with z-index makes it visible over code backgrounds, but a translucent rect over syntax-highlighted tokens washes them out: we measured dark-mode code tokens dropping from 3-6:1 to ~2:1 (keyword 1.97, comment 1.96, number 2.16, string 2.72). So a background color alone can't fix the code case, the selected text color has to be handled too.

What worked for us there: normalize the selected glyphs to one foreground color via ::selection { background-color: transparent; color: var(--foreground); -webkit-text-fill-color: var(--foreground) }, which puts the same tokens back at 5-11:1. The catch, and the reason it isn't the fix for this issue: that only works because our draw replaces the focused native selection, so there is a real ::selection to override. markSelection / selectionAlwaysOnDisplay draw when the editor is blurred, so there is no native ::selection on the text and the trick doesn't transfer. That normalization is part of the focused-selection work in #8709, not here.

Proposal: make markSelection / selectionAlwaysOnDisplay / the Extension config take a colors option instead of the hardcoded Highlight, defaulting to today's value for back-compat, plus document the occlusion + contrast story (lifting the rect makes it visible over a code background but washes the colors, and for the blurred fake selection you can't recover that with ::selection). Opened a draft PR: #8710. The colors.background part is coded (with a createDefaultOnReposition(colors) factory); making the fake selection both visible and legible over an opaque code background stays the genuine open problem. Open questions there: util vs Extension, positional arg vs options bag, and whether that visible-over-opaque-background case is worth a z-index lift here or is better left to the focused-selection path (#8709). Happy to adjust scope.