Hi yes, this is also driving me nuts. At first I thought it was a browser extension causing problems. Then I realised it was an webflow update.
I agree, the old way used to be so much easier. I very rarely use the buttons at the top to resize the canvas. This is because I design fully responsive, not adaptive, typically taking advantage of % widths where I can. Not being able to freely (without distraction) adjust the browser size is a problem when testing. Like you, I have to switch to tablet and back to desktop for it to honour the new browser width at 100%.
Equally I assumed that the new canvas resizing feature would allow me to adjust the CSS for each of the minor device breakpoints without affecting others (e.g. the 960px down from a 1170px) In the past I would export out the code and work up the minor breakpoints manually in Sublime, so I was excited at this prospect. This however, turned out not to be the case. I therefore see no real value for this new canvas resizing feature. Sorry Webflow!
Webflow, if this feature must stay, can this setting be turned off/on project by project please.