[FEATURE] Window resize speed

Prepare 2 large files for compare, 1M lines. In one file, add "UNIQUE STRING", as first or last line.

Open those files in WinMerge. Resize window (with mouse), see how fast it goes.

Ctrl + shift + D, UNIQUE STRING Now, only 1 line is shown.

Resize the window again - it is now slow.

(Unique string just shows that the number of visible lines doesn't matter.)