content area flickers with each character input after v1.19.2682.0

### Windows Terminal version

1.22.250424001

### Windows build number

10.0.26100.0

### Other Software

### Steps to reproduce

Preparation:
1. Install v1.18.10301.0 (last v1.18), v1.19.2682.0 (first v1.19)
2. set big padding(30) and grey theme(one half dark) # just easy to observe

In default windows terminal shell(Windows PowerShell):
1. Type one character
2. Observer bg color of content area and padding area
- [v1.18.10301.0] Nothing happened.
- [v1.19.2682.0] For content area, the background color darkens, then matches the padding background color after a short while.

### Expected Behavior

v1.19.2682.0 and after should same with v1.18.10301.0 and before.

### Actual Behavior

The content area flickers with each character input.

Start by reproducing the flicker in Windows Terminal v1.19.2682.0 or later with 30-pixel padding and a grey theme, then compare it with v1.18.10301.0. Trace the content-area redraw behavior during character input; done means the content background no longer darkens or changes to match the padding background.

Written by the indexing model from the issue text.