Docking "Cursors" window in the main area causes epilepsy-inducing flashing

[whitequark]

Observe:

ngscopeclient-bug-2026-05-05_03.33.19.mp4

(It's significantly worse in reality, flashing what seems like every other frame.)

Steps to reproduce:

1. NGSCOPECLIENT_UI_SCALE=3 NGSCOPECLIENT_FONT_SCALE=3 ./src/ngscopeclient/ngscopeclient
2. Add any oscilloscope
3. Right click on Waveform Group 1, Cursor > X Axis > Single
4. Dock the window next to Waveform Group 1