Split terminal: scrolling one pane while another keeps updating is extremely slow (high CPU), worse as scrollback grows #2608

Symptoms

With a split terminal layout where one pane keeps producing output, scrolling in a pane becomes extremely slow and the whole editor feels frozen.

• Scrolling a terminal split lags badly / the UI appears to hang.
• One CPU core sits near 100% the whole time.
• It gets dramatically worse the longer the terminal has been open — i.e. the more scrollback has accumulated. On a terminal that has been running for a while, a single frame can take multiple seconds.

Steps to reproduce

1. Open fresh.
2. Open a terminal and split it so there are two terminal panes side by side.
3. In one pane, run a command that prints continuously, e.g.:

   while true; do date; sleep 1; done

4. Scroll back (enter scrollback view) in one of the panes while the other keeps updating.
5. Observe: scrolling is extremely slow / the editor hangs, and CPU stays high.
6. Leave the terminal running for a while (so the scrollback grows to tens of thousands of lines) and repeat — the slowness gets much worse.

Environment

• Platform: Linux, terminal UI.