Tilemap editor color flashing on native theme

When the theme is set to Native, the background of the tilemap editor flashes, but also kind of becomes, various colors when the mouse leaves the editor area horizontally. The common colors seem to include red, white (or whatever my grid color is at the time), and teal (my system accent color).

**To Reproduce**
Steps to reproduce the behavior:
1. On a Linux installation, open a new map
2. Open preferences and under Theme set the style to Native
3. In the map, move the mouse to the side (but not the top or bottom) of the tile area onto a side panel
4. Observe a flickering overlay of an unintended color.

**Expected behavior**
the background color should not flicker, regardless of mouse position, when editing a tilemap

**Media**

expected:

**Specifications:**
- Tiled 1.11.0
- KDE Plasma 6.1.2
- Breeze theme
- Fedora 40
- Wayland

No source file or test is named. Reproduce the issue on Linux with Wayland, the Native theme, and a tilemap open, then trace the tilemap editor's background and mouse-leave handling. Done means the editor background remains stable when the pointer moves horizontally onto a side panel.

Written by the indexing model from the issue text.