Caption Max-width set incorrectly

[jkorchok]

After upgrading to 4.9.1, caption widths expressed in percentages are converted to max-width in pixels. A caption setting of width="100%" is converted to max-width="100px" in the page source. Below is the effect in both my custom theme and in the Twenty Fifteen theme.
Post text: [caption id="attachment_864" align="alignnone" width="100%"]
Page Source: <figure id="attachment_864" style="max-width: 100px" class="wp-caption alignnone">

[joyously]

When I look at the history of the img_caption_shortcode function, it has always used some form of (int) $width for the sanitization of the width, so I don't see how a percentage would ever have worked.

[jkorchok]

I've been using width="100%" since 2013 and it worked as expected until upgrading to 4.9.1.

Are you familiar with the concept of Responsive web design? Here's an introduction: ​https://www.w3schools.com/css/css_rwd_intro.asp

Responsive design requires flexibility in object sizing and fixed pixel widths don't cut it. That's why I've been using percentage widths, and it's why someone needs to fix this.

[desrosj]

Thanks for the ticket, @jkorchok! This will be addressed in #43123, so I am going to close this out as a duplicate.