using viewport relative coordinate

[tisilent]

fix #4799

[tisilent]

Stuck while writing regression tests.
general
await ctx.value.proxy.resize(20, 10); await ctx.value.proxy.resize(50, 20); await ctx.value.proxy.scrollToBottom(); await ctx.value.proxy.scrollLines(-2); await ctx.value.proxy.blur();
Scrollbar not displayed.

[Tyriar]

@tisilent I wouldn't have thought you'd need to see the scroll bar for this? Did you write enough text to full the viewport so scrollLines(-2) does something?