When composing a post with many quoted replies to other posts it is very difficult to navigate the topic and to select appropriate text. This is especially true for visually impaired users that have Text Size preference set to Largest. This is also increasingly more difficult for users on smaller screens (e.g. a 10" tablet).
Here is an example with Text Size=Large on a 10" screen (only a few lines of original text are shown on screen):
My proposal is to enable a user configurable option to dock composer window vertically instead of horizontally. This would provide for an easier navigation and quotation in such cases (if not for all users in general). Here is a mock-up:
The existing functionality (full screen composer/preview) can remain as is (composer and preview stacked side by side).