InDesign 21.6, macOS 26.6.2 (Tahoe), MacBook Pro M1 Max + two Philips 4K monitors.

The content area of UXP panels only fills about 80% of the panel: an unusable grey margin remains on the right and at the bottom, whether the panel is floating or docked, whatever its size, on all three displays, with default or scaled resolution. InDesign UI scaling is 100%.

Steps to reproduce:
1. UXP Developer Tool 2.3.0.5 > Create Plugin > InDesign quick-starter template, then Load.
2. In index.html, put in the body a red div positioned absolutely on the 4 edges (top/left/right/bottom: 0).
3. Reload and resize the panel: the red area stops at about 80% of the panel width and height.

Also: document.documentElement.clientWidth/clientHeight stay fixed (860 x 836) whatever the panel size, and window.innerWidth/innerHeight are undefined.

A community developer tested the same plugin with the same InDesign and UDT versions on macOS Sequoia 15.7 and Sonoma 14.2: no problem. It seems specific to macOS 26.

Developer forum thread: https://forums.creativeclouddeveloper.com/t/uxp-panel-content-area-does-not-fill-the-panel-grey-margin-right-bottom-indesign-21-6-macos/12398