ToT-ish lumpy build, Chrome 54.0.2806.0

Tom just introduced me to docking windows on the side of the screen. The resizing behavior seems a bit weird to me, though:

a) Start with a single non-maximized browser window.
b) Drag it by its titlebar to the right until it docks and then release[1].
c) Drag the left border of the docked window to the left.

I'm able to drag all the way to the left edge of the desktop, and the window resizes dynamically while I'm doing that, but as soon as I release the touchpad it snaps back to its maximum width, which appears to be somewhere between a quarter and a third of the display's width.

We should probably limit resizing during the drag so the window doesn't get larger than we'll actually permit it to be. I noticed that we _do_ limit how narrow it can be dragged.

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1. This is why I didn't even know that we supported docking windows. I'd seen the drag-offscreen-until-window-resizes-to-half-of-desktop-width feature, but had no idea that if I dragged farther I'd get to a different mode where the window is docked and the desktop shrinks.