Created attachment 8878591 [details] Screenshot from 2017-06-16 11-49-41.png User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0 Build ID: 20170612121707 Steps to reproduce: When using Firefox on a multi-monitor setup with both HiDPI and LoDPI screens, Firefox cannot adjust when moved between screens. Some applications built with toolkits (like GTK3) are able to (at least when using Wayland display server). Actual results: Whichever display is the "Primary" display is what Firefox uses in order to decide whether to be drawn for a HiDPI or LoDPI screen (seemingly). After that the size does not change to fit the display that it is moved to. Expected results: Firefox should be able to utilize the same technology that GTK3 does in order to adjust to accommodate the resolution of the screen it resides on. Thus displaying properly as it is moved across multiple monitors with different resolutions.
Component: Untriaged → Widget: Gtk
Product: Firefox → Core
I'm affected by this on a fresh install of Ubuntu 17.10 running Firefox 58.0 (64-bit). Machine is a Dell XPS 9360 with a HiDPI screen, connected via HDMI to a LowDPI external monitor. Firefox doesn't scale windows on the LoDPI monitor, so page elements and browser chrome appear very large. Other GTK apps such as terminal or file browser do scale correctly.
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