DOM renderer does not show selection over regular background colors

[Tyriar]

When gpuAcceleration = 'off', the yellow text background should be blue here:

[Tyriar]

Upstream: xtermjs/xterm.js#4097

[rzhao271]

The rendering's interesting.

1. Open an editor and an integrated terminal. Run the git diff command in the integrated terminal and select a large chunk of text like in the original comment.
2. Focus the editor.
3. ❗The colour of parts of the selection in the integrated terminal changes.
   

[Tyriar]

@rzhao271 good eye, I think that counts as a bug as it's certainly unexpected from my perspective. A repro emphasising what's going on is to set the color customization "terminal.inactiveSelectionBackground": "#ff0000" which results in this:

Created xtermjs/xterm.js#4736