[Bug 1770777] Re: [SRU] Bionic: PikoPixel 1.0 BETA9b can crash when resizing a document window

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** Changed in: pikopixel.app (Ubuntu)
       Status: New => Fix Released

** Also affects: pikopixel.app (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Changed in: pikopixel.app (Ubuntu Bionic)
       Status: New => In Progress

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Title:
  [SRU] Bionic: PikoPixel 1.0 BETA9b can crash when resizing a document
  window

Status in pikopixel.app package in Ubuntu:
  Fix Released
Status in pikopixel.app source package in Bionic:
  In Progress

Bug description:
  In PikoPixel 1.0 BETA9b (current Bionic version), resizing an image
  window to within a few pixels of the canvas size can cause the app to
  freeze, eventually crashing due to a stack overflow.

     A bugfix update, 1.0 BETA9c, fixes the issue, and is now in the
  Cosmic repo. Updating Bionic to Cosmic's version would prevent users
  from losing unsaved image data due to this issue.

     The crash is the result of a bug in PikoPixel's scrollview-layout
  code that causes an infinite-recursion loop of hiding & showing the
  scrollers: Window is resized -> Scrollview layout is updated -> New
  layout causes scrollers to become visible -> Scroller-visibility
  change causes re-layout of scrollview -> New layout causes scrollers
  to hide -> Scroller-visibility change causes re-layout of scrollview
  -> New layout causes scrollers to show -> etc.

  [Test Case]
  1) Start the PikoPixel app.
  2) On the "New Image" panel, click the "OK" button to create a new image (64x64 default size).
  3) The new window will display the image canvas (alternating white/grey diagonal lines with a grid-dot overlay) in the center, with a grey margin surrounding the canvas.
  4) If the bottom-right corner of the image window is covered by the "Tool Modifier Tips" panel, either click the panel's close button to hide it, or move it so the corner is visible.
  5) Move the mouse over the image window's bottom-right corner, and the mouse cursor should become a "horizontal/vertical resize" cursor (an arrow pointing towards the lower right).
  6) While the resize cursor is visible, click the corner of the window & slowly drag it towards the upper-left to make the window smaller. The canvas should stay the same size, and the grey margins should shrink (keeping the canvas centered in the window).
  7) The app-freeze happens at about the point in the resize-drag when there's no more than a pixel or two of grey margin (or no margin at all). It may take a few tries to get it to freeze, but once the margins are small, slowly moving the mouse in a circular motion should eventually trigger it. The point of moving in a circle is to cause the scrollers to alternate between becoming visible (when the window becomes smaller than the canvas), and then hiding again (when the window is larger than the canvas).
  8) When the app becomes frozen, the window itself will continue to resize in response to mouse-dragging, however the window content will stop updating, and the grey margins will no longer grow & shrink. At this point, the menus & canvas tools will no longer respond, and a system dialog may eventually appear, notifying that the window is unresponsive. (Force-quitting the app is recommended, otherwise it will eventually overflow the stack).

  [Regression Potential]
     The 1.0 BETA9c fix prevents the crash by updating the custom-layout functionality to prevent a recursive loop of showing/hiding the scrollers. It has been tested & verified that the crash no longer appears, and although no issue has been found, the most likely regression from preventing the recursive-layout loop would be that the scrollview layout would become out-of-sync with the window size. Although this is rather unlikely (layout should happen at least once nonrecursively), the unsynced-layout would just be a cosmetic issue - no stack overflow, so the app would continue to run & interact.

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